Charles Nordin

Combined renovascular hypertension and diabetes in rats: a new preparation of congestive cardiomyopathy.

Myocardial function, electrophysiologic characteristics, and structure were studied in rats with both renovascular hypertension and streptozotocin-induced diabetes (HD). Ventricular papillary muscles from untreated rats with HD showed a marked slowing of isometric and isotonic contractions. Peak developed tension and peak shortening were preserved, except in one animal with findings of congestive heart failure. Transmembrane action potentials increased fivefold in duration. Myocardial interstitial fibrosis was frequently observed. Physiologic parameters of rats with HD treated by left nephrectomy, captopril, and insulin were very similar to those of age-matched controls. The mortality rate of rats with HD was 43% over 5 to 6 months in the first study. In a second study, spontaneously dying rats with HD were compared with those deliberately killed. A 55% mortality was observed over 7 months. Myocardial structural damage and histologic evidence of congestive heart failure were more frequent in spontaneously dying rats with HD. Combined renovascular hypertension and diabetes in rats appears to be a new preparation of congestive cardiomyopathy.

Terfenadine blocks time-dependent Ca2+, Na+, and K+ channels in guinea pig ventricular myocytes.

Terfenadine, which blocks delayed rectifier K+ channels (Ik), is structurally related to diphenylalkylamine L-type Ca2+ channel (ICa) blockers and has been reported to render Purkinje fibers inexcitable. We used standard whole-cell patch clamp techniques in isolated guinea pig ventricular myocytes to investigate the direct effect of terfenadine on ICa after discovering that the upstrokes of early afterdepolarizations in guinea pig myocytes were inhibited by the drug at concentrations > or = 10(-6)M. Some data analyzing the effect of terfenadine on time-dependent Na+ channels (INa) and IK also were obtained. All experiments were controlled for time of intracellular dialysis. Terfenadine (3 x 10(-6)M) reduced peak ICa (measured in either K+-containing or Cs+-substituted intracellular solutions from holding potentials of -40 mV) after 10 min exposure [peak at 0 mV in K+-deficient dialysis solution -4.2 +/- 2.3 pA/pF (mean +/- SD, n = 5) versus -13.02 +/- 4.33 pA/pF in control solution (n = 5), p < 0.01], and ICa was almost completely blocked after 15 min drug exposure. Ten minutes of exposure to terfenadine (3 x 10-6M) also caused near-complete blockade of peak INa when INa was measured at -40 mV after 300 ms conditioning pulses from a holding potential of -40 to potentials between -60 and -90 mV. The effect was much less pronounced when INa was measured from a holding potential of -90 mV. After exposure to terfenadine 3 x 10 (-6)M, IK density, measured as peak tail current at -40 mV after 300-ms depolarizations, was also reduced but not eliminated at membrane potentials between -20 and +60 mV. In contrast, exposure to terfenadine caused no significant change in the current-voltage relationship after 300-ms steps from -90 to +60 mV. Terfenadine had no effect on time constants of decay of IK or ICa. These results suggest that terfenadine blocks several time- and voltage-dependent channels, possibly by binding to a common protein structure, not related to ion selectivity, that is primarily associated with time-dependent activation of channel conductance.

Electrophysiologic characteristics of single myocytes isolated from hypertrophied guinea-pig hearts

To determine whether the electrical changes associated with cardiac hypertrophy are due to alterations in the membrane properties of individual hypertrophied cells, we recorded action potentials in single myocytes isolated from normal and hypertrophied hearts. Cardiac hypertrophy was produced by a gradual pressure overload created by placing a band around the ascending aorta in young guinea-pigs (200-250 g). Almost half the animals that developed left ventricular (LHV) hypertrophy also developed evidence of cardiac dysfunction. Action potentials were recorded with standard microelectrodes in single ventricular myocytes isolated by enzymatic dispersion of the heart. The action potential duration at 1 Hz was significantly longer in hypertrophied cells than in control cells. The degree of action potential prolongation in isolated cells did not correlate with the degree of hypertrophy but did correlate with the degree of myocardial disease, the duration being longer in hypertrophied myocytes from dyspneic than in those from non-dyspneic animals. The resting potential was significantly lower in hypertrophied myocytes from dyspneic animals than in hypertrophied cells from non-dyspneic animals or control cells stimulated at 5 Hz. The relationship between the frequency of stimulation (0.33, 1, and 5 Hz) and action potential duration was steeper in hypertrophied than normal myocytes. The mean membrane capacitance (cm) of hypertrophied myocytes increased by 31% over the control value. Thus, isolated hypertrophied myocytes retain the prolonged duration of the action potential and the exaggerated dependence of duration on rate observed in intact hypertrophied muscle. The increased duration of the action potential in hypertrophied cells cannot be readily attributed to the observed increase in cm. Our results indicate that the membrane changes responsible for the altered electrical properties of hypertrophied myocardium are due to an effect of hypertrophy on individual myocytes and that the prolonged duration of the action potential is probably due to changes in active currents flowing during repolarization.

Altered myocardial response to ouabain in diabetic rats: Mechanics and electrophysiology

Diabetes induced by streptozotocin in rats is associated with changes in the mechanical function of isolated ventricular papillary muscle. Relaxation is slowed and shortening velocity is depressed. The effects of ouabain (10(-7) to 2 x 10(-4)M) and changes in extracellular calcium ([Ca2+]0 = 0.6 to 12 mM) on the mechanical and electrical properties of normal and diabetic papillary muscles were studied. High doses of ouabain caused a rise in resting tension and a fall in developed tension in both diabetic and control muscles. However, these changes were strikingly greater in diabetic muscles which developed partial contractures at 10(-4)M. The altered response to ouabain was observed in chronically (5 to 7 weeks or 3 months) but not acutely (less than 1 week) diabetic animals. Similarly, the response to ouabain was restored to normal after chronic (5 weeks) therapy with insulin but not after acute (4 days) therapy. In both normal and diabetic muscles, the mechanical effects of increasing [Ca2+]0 from 2.4 to 12.0 mM were qualitatively similar to those seen with ouabain at 10(-5) to 10(-4)M. Electrophysiologic studies showed that under control conditions action potentials of diabetic muscles were significantly longer than those of normal muscles. Treatment with progressively higher concentrations of ouabain (10(-7) to 10(-4)M) and [Ca2+]0 (2.4 to 12.0 mM) caused shortening of both normal and diabetic action potentials, but the effects of these interventions were much greater in the diabetics. These results suggest that the response of diabetic muscles to ouabain is markedly different from normal and that this altered response may be due to impaired regulation of intracellular Ca2+ levels in diabetic myocardium.

Prevalence of Hemoglobin A1c Greater Than 6.5% and 7.0% among Hospitalized Patients without Known Diagnosis of Diabetes at an Urban Inner City Hospital

CONTEXT Bronx, New York, an urban county with a large low-income, immigrant and minority population, has a prevalence of diabetes that is among the highest in the United States. OBJECTIVE The aim of the study was to evaluate the utility of hemoglobin A1c (HbA1c) in identifying patients at risk for diabetes on an in-patient medical service of a hospital serving a high prevalence community. DESIGN AND SETTING We conducted a prospective cohort study at an urban public hospital. PATIENTS The study included 971 patients (1132 admissions) admitted to the general medicine service over 4 months. MAIN OUTCOME MEASURES HbA1c was measured on all patients. Records were checked for prior diagnosis of diabetes and other clinical data. Follow-up data were obtained for those with repeat HbA1c testing or glucose within 1 yr after admission. RESULTS We found that 35.2% of the patients (n = 342) had an established diagnosis of diabetes. The remaining 629 patients defined the study cohort of patients without known diabetes. Mean HbA1c was 6.05 +/- 0.87%. A total of 152 patients (24%) had admission HbA1c of at least 6.5% and 62 (9.9%) had HbA1c of at least 7.0%. Fifty-five patients with HbA1c of at least 6.5% had follow-up HbA1c within 1 yr. Of those, 44 (80.0%) met the criteria for diabetes as proposed by The International Expert Committee using repeated HbA1c testing. CONCLUSION In communities with high prevalence of diabetes, a large percentage of patients without a diagnosis of diabetes who are admitted as in-patients have HbA1c of at least 6.5% and 7.0%. Hospital-based HbA1c testing might identify patients for whom further testing is indicated to make the diagnosis of diabetes.

Diabetes and stress hyperglycemia associated with myocardial infarctions at an urban municipal hospital: Prevalence and effect on mortality

Municipal hospitals in large cities provide care for patients from immigrant and mixed ethnic communities that are at high risk for diabetes. Both diabetes and stress hyperglycemia increase the risk of adverse outcome after myocardial infarctions, and the impact of stress hyperglycemia on the outcome of myocardial infarctions in this particular setting has not been previously studied. We therefore undertook a retrospective cohort study to determine the prevalence of diabetes and stress hyperglycemia in patients presenting to a university-affiliated Bronx municipal hospital with myocardial infarction, and the relationship of these conditions to the extent of coronary disease and mortality. We obtained data on 106 consecutive patients from July 1998 to April 1999 with a diagnosis-related group diagnosis of either myocardial infarction or acute coronary syndrome, in which myocardial infarction was confirmed by serum enzymes or characteristic electrocardiographic changes. Patients were followed until March 30, 2001. Measurements of clinical parameters and results of catheterization were obtained for all patients. Death rates were determined by laboratory database, direct patient contact, or data from National Death Index. Eighty percent of the cohort had either a diagnosis of diabetes (n = 45, 42% of cohort) or evidence of stress hyperglycemia (defined as serum glucose greater than 126 mg/dL at the time of admission without prior diagnosis of diabetes, n = 40, 38%). In-hospital mortality for patients with diabetes, stress hyperglycemia, or normal glucose was 20%, 15%, and 14%, respectively. Eighty-three percent of the cohort received beta blockers, and 61% of hospital survivors had catheterization. Left main or triple vessel disease was common in both patients with diabetes (52%) and patients with stress hyperglycemia (32%). Mortality at follow up (maximum follow up 3 years; mean follow up 19.6 months) was much higher in patients with either diabetes (42%) or stress hyperglycemia (52%) than normal subjects (24%). Kaplan-Meier analysis of the difference in mortality between patients with high glucose on admission and normal subjects was borderline significant (P = 0.06). Multivariate regression demonstrated that age (P = 0.020), increase in admission serum creatinine (P = 0.001), and reduction in either ejection fraction (P = 0.016) or admission systolic blood pressure (P = 0.005) were significant predictors of mortality. Glycemic status and sex were not independently associated with death after controlling for these other factors. These results show that the prevalence of both diabetes and stress hyperglycemia on presentation with myocardial infarction is strikingly high in this immigrant, mixed ethnic, urban population. Patients with diabetes and stress hyperglycemia had advanced disease on presentation and much higher mortality at 2 to 3 years than those with normal blood glucose. The mortality difference is the result of older age and more advanced disease rather than hyperglycemia per se.

The proarrhythmic effect of hypoglycemia: evidence for increased risk from ischemia and bradycardia

Hypoglycemia increases the risk for both overall and sudden death. At a cellular level, hypoglycemia causes alterations in the physiology of myocardial tissue that are identical to proarrhythmic medications. Reduced serum glucose blocks the repolarizing K+ channel HERG, which leads to action potential and QT prolongation and is uniformly associated with risk for torsades de pointes ventricular tachycardia. The sympathetic response induced by hypoglycemia also increases the risk of arrhythmias from Ca2+ overload, which occur with sympathomimetic medications and excessive beta adrenergic stimulation. Thus, hypoglycemia can be considered a proarrhythmic event. This review focuses on emerging evidence for two other important changes induced by hypoglycemia that promote arrhythmias: ischemia and bradycardia. Studies of patients with “insulin shock” therapy from the early twentieth century and other more recent data strongly suggest that hypoglycemia can cause ischemia of myocardial tissue, both in association with coronary artery obstructions and by cellular mechanisms. Ischemia induces multiple proarrhythmic responses. Since ischemia itself reduces the possibility of using energy substrates other than glucose, hypoglycemia may generate positive feedback for electrophyisologic destabilization. Recent studies also show that hypoglycemia can cause bradycardia and heart block. Bradycardia is known to cause action potential prolongation and potentiate the development of torsades de pointes, particularly with low-serum K+ which can be induced by hypoglycemic episodes. Thus, hypoglycemia-induced bradycardia may also create a dynamic, positive feedback for the development of arrhythmias and sudden death. These studies further support the hypothesis that hypoglycemia is a proarrhythmic event.

Arrhythmogenic interaction between low potassium and ouabain in isolated guinea-pig ventricular myocytes.

1. The two‐microelectrode method of voltage clamp was used in single myocytes isolated from guinea‐pig ventricles to investigate the mechanism underlying the arrhythmogenic interaction between low external K+ and digitalis. 2. We investigated the effects of ouabain (10(‐6) M) with 4 mM‐K+ or 2 mM‐K+ on the peak magnitude of the inward component of oscillatory current (Iti) recorded upon repolarization to the resting potential after depolarizing clamps to ‐5 mV, and on the characteristics of the steady‐state current‐voltage relationship. 3. Whereas ouabain with 4 mM‐K+ did not alter the current‐voltage relationship from its control shape, ouabain with 2 mM‐K+ caused marked changes in the curve: the zero‐current intercept was shifted in a negative direction, the region of low slope conductance was extended to more negative potentials, and the curve was shifted downward relative to the control current‐voltage relationship. The changes in the current‐voltage relationship induced by ouabain with 2 mM‐K+ were very similar to those induced by 2 mM‐K+ alone. 4. The functional consequence of the changes in the current‐voltage relationship induced by ouabain with 2 mM‐K+ was a highly significant reduction in the amount of outward current (Ith) needed to reach the threshold for excitation; Ith was reduced from 1.6 +/‐ 0.4 nA (n = 6) in ouabain with 4 mM‐K+ to 0.8 +/‐ 0.3 nA (n = 5) in ouabain with 2 mM‐K+. 5. The mean value for Iti was larger in ouabain with 2 mM‐K+ (0.58 +/‐ 0.41 nA, mean +/‐ S.D., n = 4) than in ouabain with 4 mM‐K+ (0.42 +/‐ 0.38 nA, n = 5). Although the increase in Iti was not statistically significant because of the large variability of the measurement, it is possible that the increase might be physiologically significant. 6. Our results suggest that the arrhythmogenic interaction between digitalis and low K+ is due to the combined effects of low K+ on the current‐voltage relationship and on the size of the peak inward current induced by ouabain. Whereas the effect of low K+ and ouabain on the inward current was highly variable, the effects on the current‐voltage curve were far more consistent, pointing to an important role for alterations in the current‐voltage relationship in the arrhythmogenic interaction between low K+ and ouabain.

Hyperlipoproteinemia in spontaneously diabetic guinea pigs

A colony of Hartley guinea pigs that exhibit hyperglycemia, glucosuria, and hypertriglyceridemia characteristic of human diabetes mellitus was developed. Initially, a group of guinea pigs that had normal serum glucose concentrations (less than or equal to 200 mg/dL of serum) at 3 to 4 weeks of age was obtained; however, in some of the animals progressively severe hyperglycemia (300 to 500 mg/dL of serum) and glucosuria (greater than 2 g of glucose/24 h) occurred as the animals matured. In addition, the animals exhibiting hyperglycemia and glucosuria had plasma insulin concentrations that were similar to those animals that were not hyperglycemic. The diabetic animals were found to be hypertriglyceridemic, with plasma triglyceride levels of 140 to 290 mg/dL at four months of age. Nondiabetic animals (plasma glucose concentration of less than or equal to 200 mg/dL and no glucosuria) had plasma triglyceride concentrations between 37 and 76 mg/dL. Lipoprotein analysis of plasma from nondiabetic and diabetic animals indicated that the diabetics had a fourfold increase in VLDL triglyceride and protein concentrations. The VLDL had an abnormal apolipoprotein composition and had reduced levels of apoprotein-E. The progeny from the mating of diabetic males and females also exhibited the diabetic trait, suggesting that the origin of the disease is genetic. This colony of guinea pigs is being further investigated as a suitable model for the study of the hyperlipoproteinemia of human noninsulin-dependent diabetes mellitus.

The role of reduced potassium conductance in generating triggered activity in guinea-pig ventricular muscle

This study investigates the role of reducing potassium conductance (gK) in generating delayed afterdepolarizations and triggered activity in small preparations of ventricular muscle from guinea-pig hearts. We used agents believed to reduce gK (low or absent K0, tetraethylammonium (TEA), CsCl) and we used ouabain (10(-6) M) to induce delayed afterdepolarizations. Treatment with ouabain only caused subthreshold delayed afterdepolarizations or occasionally non-sustained triggered activity. Exposure to Tyrodes solution with K reduced from 4 to 2 mM or K-free Tyrodes solution, with or without ouabain, caused subthreshold delayed afterdepolarizations and sometimes non-sustained triggered activity. Exposure to Tyrodes solution containing TEA and ouabain caused sustained triggered activity, supporting the hypothesis that accumulation of extracellular K inhibits the development of triggered activity. Presumably, the reduction in gK caused by TEA is not reversed by accumulation of extracellular K so that the delayed afterdepolarizations in the presence of persistently reduced gK are large enough to induce sustained triggered activity. Under extreme conditions, when Cs replaced K and half the NaCl was replaced by TEA, delayed afterdepolarizations occurred in the presence of markedly reduced gK, the result being the rapid development of sustained triggered activity, even at the basic drive rate of 1 Hz. Our results suggest that reduced gK plays an important role in the development of triggered activity.