K. L. Kam

The Effects of Hypertension and Diabetes Mellitus on the Vascular Reactivity of Resistance Arteries

We have studied the effects of both hypertension and streptozotocin-induced diabetes mellitus on alpha 1-adrenoceptor mediated vasoconstriction, endothelium-dependent and endothelium-independent vasodilation. The experiments were performed in perfused mesenteric vascular bed preparations taken from age-matched SHR, WKY, diabetic SHR and diabetic WKY. For the alpha 1-adrenoceptor agonist methoxamine, the mesenteric preparations from SHR and diabetic SHR yielded significantly (p < 0.05) stronger maximal responses than preparations taken from WKY and diabetic WKY, respectively. The diabetic state significantly (p < 0.05) decreased the responsiveness to methoxamine in arteries from SHR and WKY. Hypertension does not significantly change the concentration response-curves for (acetyl-beta) methacholine, histamine, adenosine diphosphate and sodium-nitroprusside. However, the sensitivity to endothelium-dependent vasodilation decreased in preparations from diabetic animals (< 0.05). It is concluded that mesenteric resistance arteries from SHR and diabetic SHR are more reactive to alpha 1-adrenoceptor stimulation, whereas diabetes reduces the responsiveness to methoxamine in WKY and SHR. Hypertension does not affect the endothelium-dependent relaxation in mesenteric arteries. However, diabetes decreases the sensitivity to endothelium-dependent relaxation without altering the sensitivity to sodium-nitroprusside. These findings are indicative of a diabetes-induced endothelial dysfunction in mesenteric resistance arteries. In preparations from diabetic hypertensive rats the reduced response to methoxamine and the endothelial dysfunction seem to run parallel.

Contractile responses to various stimuli in isolated resistance vessels from simultaneously hypertensive and streptozotocin-diabetic rats.

Diabetes mellitus and hypertension are common chronic diseases that frequently occur simultaneously. The induction of streptozotocin (STZ) diabetes mellitus in spontaneously hypertensive rats (SHR) offers the opportunity to investigate the influence of both entities in a reproducible manner. We investigated the effects of various vasoconstrictors on isolated small arteries from the mesenteric vascular bed of normotensive rats (Wistar-Kyoto rats, WKY) and SHR with chronic (8 weeks), STZ-induced diabetes mellitus. No consistent changes in hemodynamic parameters of the (STZ-) normotensive and (STZ-) hypertensive rats were noted. The K(+)-normalization procedure yields the individual optimal lumen diameter, which was the same for the arteries of the four groups of rats. The passive wall tension resulting from this normalization procedure was higher only in preparations from the control hypertensive group as compared with those from the control normotensive rats. Morphological investigations showed that small arteries from control SHR had an increased tunica media thickness as compared with those of control WKY; the STZ-WKY had an increased tunica media thickness as compared with preparations from control WKY. The vasoconstriction caused by alpha 1-adrenoceptor stimulation [norepinephrine (NE), methoxamine] and serotonin is unchanged in chronic experimental diabetes. The diabetic state reduced the sensitivity [-log EC50(M)] for the concentration-response curves (CRC) of calcium chloride. The CRC of potassium chloride indicated the same sensitivities, but maximal active wall tensions of vessels from STZ-SHR were reduced as compared with those from STZ-WKY. The well-known enhancement of the effects of various contractile stimuli caused by hypertension could not be demonstrated for the isolated small arteries used in the present study, although a nonsignificant tendency was observed. However, the STZ-diabetic state did not cause important additional pharmacodynamic changes, despite the morphological alterations in those vessels.

Cardiac iodine-123 metaiodobenzylguanidine uptake in animals with diabetes mellitus and/or hypertension

The aim of the present study was to evaluate the use of the noradrenaline analogue iodine-123 metaiodobenzylguanidine ([123I]MIBG) for the assessment of cardiac sympathetic activity in the presence of diabetes mellitus and/or hypertension in animal models. One model used Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) rendered diabetic at 12 weeks of age by an intravenous injection of streptozotocin (STZ). The other model used lean and obese Zucker rats. In all groups basic haemodynamic values were established and animals received an intravenous injection of 50 μCi [123I]MIBG. Initial myocardial uptake and washout rates of [123I]MIBG were measured scintigraphically during 4 h. After sacrifice, plasma noradrenaline and left cardiac ventricular β-adrenoceptor density was determined. The diabetic state, both in STZ-treated rats (direct induction) and in obese Zucker rats (genetic induction), appeared to induce a lower cardiac density of β-adrenoceptors, indicative of increased sympathetic activity. Cardiac [123I]MIBG then showed increased washouts, thereby confirming enhanced noradrenergic activity. This parallism of results led to the conclusion that [123I]MIBG wash-out measurements could provide an excellent tool to assess cardiac sympathetic activity non-invasively. However, in hypertension (WKY vs SHR), both parameters failed to show parallelism: no changes in β-adrenoceptor density were found, whereas [123I]MIBG wash-out rate was increased. Thus, either [123I]MIBG washout or ß-adrenoceptor density may not be a reliable parameter under all circumstances to detect changes in the release of noradrenaline. Changes in the initial uptake of [123I]MIBG were observed as well. This may be a good marker for the disappearance of cardiac innervation, but it seems not to be a good parameter for distinguishing between loss of sympathetic innervation and enhanced uptake of noradrenaline in pathological conditions.

Streptozotocin-induced diabetes mellitus in spontaneously hypertensive rats: A pathophysiological model for the combined effects of eypertension and diabetes

Abstract The present study was undertaken to investigate the combined effects of hypertension and streptozotocin-induced diabetes mellitus in the rat. Accordingly, four groups of rats were studied: Wistar Kyoto rats (WKY), diabetic WKY, spontaneously hypertensive rats (SHR) and diabetic SHR, respectively. The mean arterial blood pressure was increased in hypertensive animals compared to normotensive animals. The base excess in the diabetic rats was higher than that of normoglycemic animals. An elevated glucose concentration was found in the blood and urine of streptozotocin-treated rats. Ketone bodies were detected in the urine and blood of the diabetic rats. Mortality rates after treatment were not different among the four groups. In separate experiments, isolated working hearts of the various groups were set up and analyzed. For the maximal left ventricular pressure (mm Hg) the following values were formed: 110.0 ± 2.6, 93.6 ± 2.7, 93.4 ± 3.0, and 87.5 ± 2.4, respectively. The wet heart weights, dry heart weights, and body weights of the diabetic rats were lower than those of normoglycemic animals. The wet heart weight/body weight ratio, however, was increased by diabetes and hypertension (0.43 ± 0.01, 0.47 ± 0.01, 0.47 ± 0.01, and 0.54 ± 0.02, respectively). There were no significant differences between the water content of the hearts from the four different groups. Pathologic examination of the hearts showed myocardial hypertrophy and medial hypertrophy of coronary arteries in diabetic and hypertensive animals. There was no difference in relative collagen content in the hearts of the four groups. In conclusion, the experimental procedure used appears to be an unique model to study the effects of diabetes and hypertension separated as well as combined.

Drug-Induced Endothelium-Dependent and -Independent Relaxations in Isolated Resistance Vessels Taken from Simultaneously Hypertensive and Streptozotocin-Diabetic Rats

Hypertension and diabetes mellitus often co-exist and both conditions may be expected to cause synergistic vascular damage. Our group has introduced an animal model for simultaneously occurring hypertension and diabetes mellitus by treating spontaneously hypertensive rats with streptozotocin (STZ). We investigated the drug-induced endothelium-independent and -dependent relaxation in isolated mesenteric small arteries (resistance vessels). Concerning the influence of hypertension, the responses to sodium nitroprusside, methacholine, histamine and nifedipine proved unchanged, the vasodilator response to bradykinin was diminished, whereas that to the K(+)-channel opener cromakalim was enhanced. With respect to the influence of STZ-induced diabetes we found that the responses to sodium nitroprusside, methacholine and nifedipine were unchanged, and that to cromakalim was enhanced, also when the preparations were pretreated with glibenclamide. The responses to histamine (STZ WKY versus control WKY) and bradykinin (STZ SHR versus control SHR) proved enhanced in the isolated vessels taken from diabetic animals. These findings suggest that the influence of the diabetic state is more pronounced than that of hypertension. However, our findings do not indicate that either hypertension or diabetes is associated with generalised endothelial damage in the resistance arteries.

Antiischemic effects of nifedipine in isolated working heart preparations of healthy, diabetic, and hypertensive rats

We evaluated the antiischemic effects of nifedipine in isolated working rat hearts from age-matched normotensive Wistar-Kyoto rats (WKY), diabetic WKY, spontaneously hypertensive rats (SHR), and diabetic SHR. Diabetes was induced by streptozotocin. First, we constructed concentration-response curves for the negative inotropic effect of nifedipine in every group. After 15 min of pretreatment with nifedipine (EC60), low-flow ischemia (30 min) was induced by reducing the afterload from 51.5 to 11.0 mm Hg and nifedipine was infused simultaneously. The six measured parameters were left ventricular pressure (LVP), maximum rate of pressure increase (+dP/dtmax), maximum rate of pressure decrease (-dP/dtmax), aortic output (AO), coronary flow (CF), and cardiac output (CO), determined after 15-min equilibration in the working heart mode and at the end of the experiment. From these data, the recovery percentages were calculated. There were no significant differences in sensitivity to nifedipine (as measured by the EC50 concentration) between the four groups with respect to LVP, +dP/dtmax, -dP/dtmax, CF, and CO. However, hearts from SHR were less sensitive to nifedipine than those from diabetic SHR and nondiabetic WKY with regard to AO. In isolated hearts from nondiabetic WKY and SHR, there were no significant differences between vehicle-treated organs and nifedipine-treated preparations. In hearts from diabetic WKY and diabetic SHR, however, the nifedipine-treated group (LVP 87.1 +/- 3.3 and 60.5 +/- 12.1%, respectively) recovered significantly (p < 0.05) better from ischemia as compared with the control group (LVP 35.7 +/- 14.7 and 10.7 +/- 9.8%, respectively) (n = 6 for each group).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacodynamic behaviour of isolated resistance vessels obtained from hypertensive‐diabetic rats

Summary— The influence of various vasodilator and constrictor drugs was studied in isolated mesenteric arteries obtained from obese and lean Zucker rats. The obese Zucker rats were moderately hypertensive and their isolated small arteries were not hypertrophied. All vasoconstrictor agents studied (noradrenaline, methoxamine, serotonin, calcium chloride, potassium chloride) caused the same effects in isolated arteries taken from obese and lean (control) Zucker rats, respectively. In vessels from obese Zucker rats, the vasodilator responses to sodium nitroprusside, methacholine, the K+‐channel opener cromakalim and nifedipine were the same as in control preparations. In conclusion, isolated mesenteric arteries from obese Zucker rats do not show relevant structural changes, and the pharmacodynamic behaviour of such vessels appears to be the same as that of control preparations. Neither the diabetic hyperinsulinaemic state nor the hyperlipoproteinaemia in the obese Zucker rats appear to cause serious vascular damage.

Effects of R 56865 on postischemic ventricular function in isolated rat working heart preparations obtained from healthy, diabetic and hypertensive animals.

The present study was undertaken to evaluate the effects of R 56865 (N-[1-[4-(4-fluorophenoxy)-butyl]-4-piperidinyl-N-methyl-2-benzothiazolamine) (Fig. 1) on postischemic ventricular function, an inhibitor of the Na+/Ca2+ overload, in the working heart preparation of the rat. The hearts were paced at 5 Hz and perfused with Tyrode solution of 37°C at a physiological pH.After 15 min of pretreatment with R 56865, low-flow ischemia (30 min) was induced by reducing the perfusion pressure from 51.5 mmHg to 11.0 mmHg and R 56865 was infused simultaneously. The hemodynamic effects of R 56865 were evaluated in the concentration range [10−8-3·10−6M]. The five parameters measured were: LVP (Left Ventricular Pressure), +dP/dtmax (maximal rate of pressure increase), AO (Aortic Output), CF (Coronary Flow) and CO (Cardiac Output). They were determined in the working heart mode after 15 min of equilibration and at the end of the experiment. From these data the recovery percentages were calculated. The recovery percentages for the LVP, +dP/dtmax, AO, CF and CO for the control hearts (3.3%, 0.0%, 7.9%, 10.4% and 8.5%, respectively) differed significantly from those at 10−7 M (39.6%, 40.8%, 25.0%, 41.8% and 29.9% respectively). The recovery percentage were the highest at 10−6 M (79.6%, 82.1%, 54.7%, 92.7% and 67.2%, respectively). The concentration of 10−7 M was associated with a smaller reduction in LVP (12.9%) than at ·10−6 M (25.7%).In separate experiments the hemodynamic effects of R 56865 were investigated in working hearts taken from age-matched normotensive Wistar Kyoto rats (WKY), diabetic WKY, spontaneously hypertensive rats (SHR) and diabetic SHR. Diabetes was induced by intravenous (i.v.) Streptozotocin (60 mg/kg), administered at 12 weeks of age. The diabetic animals were kept diabetic for 8 weeks. In this stage there was no difference in mortality rate between the 4 groups. In the four groups studied the R 56865 concentration was ·10−6 M, which had been found as optimal in the experiments with preparations from normal Wistar rats. The recovery percentages with R 56865 for the LVP in hearts derived from the WKY, diabetic WKY, SHR and diabetic SHR were: 88.8%, 82.0%, 49.8% and 53.2% respectively. These percentages were significantly different from the data derived from the vehicle treated hearts (14.6%, 35.7%, 3.2%, 10.7%, respectively). Similar differences also hold true for the other parameters. Accordingly, R 56865 also improves postischemic ventricular function significantly under pathologic conditions, where the recovery of the hearts from SHR was improved less than that of WKY.

The influence of high glucose levels and/or hyperosmolarity on rat isolated aorta

We investigated the influence of high-glucose levels (30.5 mmol/L) and/or hyperosmolarity on pharmacological responses in aortic ring preparations taken from nondiabetic rats. Moderate changes were observed for the concentration-response curves of noradrenaline and phenylephrine, but not for methoxamine and cirazoline. Maximal active forces of the concentration-response curves of potassium chloride and calcium chloride were significantly reduced by the elevated glucose levels per se, but neither the slopes of the curves nor the -logEC50 values were affected. Concentration-response curves of serotonin and U 46619 were not affected. For angiotensin II, the -logEC50 values and maximal active forces of the concentration-response curves were significantly lower under both hyperglycemic and hyperosmolar conditions. The present study suggests that potential changes in contractile behavior in isolated vessels from diabetic animals cannot be attributed to high glucose levels and/or hyperosmolarity as such, but indeed reflect vascular changes associated with the diabetic state. An exception has to be made for the depolarization of aortic ring preparations of nondiabetic rat; the elevated glucose level leads to an impaired calcium influx via the (slow) L-type calcium channels.