W. Vleeming

Effect of nitrite on blood pressure in anaesthetized and free-moving rats

The effect of nitrite on blood pressure and heart rate was studied in anaesthetized (non-telemetric method) and free-moving rats (biotelemetry system). In anaesthetized rats, NaNO2 (10-1000 mumol/kg), infused over 5 min, induced a dose-related decrease in blood pressure. The maximal decrease in mean arterial blood pressure (MAP), caused by 1000 mumol/kg NaNO2 and measured 15 min after infusion was 55.9 +/- 3.2% (n = 3). After NaNO2 infusion, in the plasma, rapid conversion of nitrite into nitrate was observed. However, sodium nitrate (NaNO3, 100 mumol/kg) did not decrease blood pressure and there was no conversion of nitrate into nitrite. Free-moving rats received KNO2 which was added to drinking water (36 mmol/litre) for a period of 3 days. KNO2 decreased the MAP and increased the heart rate during the rats activity phase at night but not during their resting phase in the day. An equal concentration of potassium (KCl, 36 mmol/litre added to drinking water) for 3 days did not decrease blood pressure. It is concluded that nitrite decreases blood pressure in rats, which probably induces, by renin-angiotensin system activation, hypertrophy of the adrenal zona glomerulosa.

Vascular responses of isolated mesenteric resistance and basilar arteries from short- and long-term diabetic rats

Vascular dysfunctions, e.g. alterations in the reactivity of blood vessels to neurotransmitters and hormones, are a well-established complication of diabetes mellitus. Whether these impairments are a consequence of direct postsynaptic deficits and/or indirect presynaptic deficits remains to be determined. To this end, we investigated the influence of the duration of diabetes on relaxation and contraction responses of isolated mesenteric resistance and equally-sized basilar arteries to postsynaptic activation by various vasoactive agents, using streptozotocin-induced diabetic rats and age-matched controls. Relaxation responses to vasodilator agents were studied in KCl-precontracted arteries. The duration of diabetes (4 or 40 weeks) did not affect the vasodilator responses to sodium nitroprusside or salbutamol in either artery. In mesenteric resistance vessels from short-term (4 weeks) and long-term (40 weeks) diabetic rats the vasoconstrictor responses to KCl, serotonin and vasopressin were the same as those in non-diabetic rats; however, the sensitivity (EC50) to noradrenaline was slightly but significantly enhanced after the long-term diabetic state. In contrast to the mesenteric arteries, noradrenaline did not cause contraction in basilar arteries taken from diabetic and control rats. Thus, there appear to be important differences in the reactivity to noradrenaline of the peripheral and cerebral vasculature. The basilar artery from short-term and long-term diabetic rats did not show different responsiveness to vasopressin whereas to serotonin a significant enhanced and decreased sensitivity (EC10 and EC50) was demonstrated in short-term and long-term diabetes, respectively. Our findings indicate that postsynaptic impairments do not play a major role in the alterations of vasoreactivity to vasodilators, noradrenaline or vasopressin seen in experimental diabetes. However, the duration of the diabetic state may have serious consequences for vasoreactivity of basilar arteries to serotonin and, therefore, warrants further investigations.

Experimental study of the detrimental effect of dopamine/glucagon combination in d,l- propranolol intoxication

1 Respiratory and cardiovascular failure are the princi ple toxic effects of β-blocker overdose. Respiratory arrest is the primary cause of death in β-blocker intoxicated rats. 2 The effect of glucagon, dopamine and the combination of glucagon/dopamine on respiratory and cardiovas cular function and survival time in β-blocker overdose was investigated in a model of acute d,l-propranolol (resp. 30 and 15 mg kg-1 h-1 in rat and rabbit) intoxication in spontaneously breathing rats and artifically ventilated rats and rabbits. 3 Glucagon (initial dose of 100 μg kg-1 (bolus), followed by 1 μg kg -1 min-1), dopamine (25 μg kg-1 min-1 ) or the combination of glucagon/dopamine did not im prove survival time (ST) in d,l-propranolol intoxicated spontaneously breathing rats and artificially venti lated rats and rabbits, although some haemodynamic variables i.e. heart rate (HR), mean arterial blood pressure (MAP), left ventricular pressure (LVPmax) and the differentiated left ventricular pressure (LVdp/ dtmax) temporarily improved. 4 Survival time was considerably reduced in d,l- propranolol intoxicated spontaneously breathing and artifically ventilated rats treated with a combination of glucagon /dopamine, which induced a decrease in PaO2 and pH and an increase in PaCO2 partly due to ventilation/perfusion mismatch. 5 The combination of glucagon/dopamine should be used carefully in the treatment of β-blocker overdose in man.

Reduced survival after isoprenaline/ dopamine in d,l-propranolol intoxicated rats:

1 Respiratory and cardiovascular failure are principle toxic effects of β-blocker overdose. Respiratory arrest is the primary cause of death in β-blocker intoxicated rats. 2 The effect of β-adrenoceptor agonists on respiratory and cardiovascular failure in β-blocker overdose was investigated in a model of acute d,l-propranolol (30 mg kg -1 h-1) intoxication in spontaneously breathing rats. 3 Neither the aselective, hydrophilic β-agonist isoprena line (10, 25, 50 μg kg-1 min-1), nor the β 1-selective, lipophilic β-agonist flerobuterol (1,3,10 μ g kg-1 min-1) and the β2-selective, lipophilic β-agonist clenbuterol (10, 25, 50 μg kg-1 min-1) had any beneficial effect on cardiovascular and respiratory variables or survival time in d,l-propranolol intoxicated spontaneously breathing rats. 4 Isoprenaline (10 μg kg-1 min-1) had no favourable effect on haemodynamic and respiratory variables in artificially ventilated d,l -propranolol intoxicated rats either. 5 Addition of dopamine to isoprenaline resulted in a significant reduction of survival time, primarily caused by a decrease in mean arterial blood pressure, in artificially ventilated d,l-propranolol intoxicated rats. Addition of glucagon to isoprenaline did not affect survival time. 6 Artificial ventilation is the most important supportive measure in d,l-propranolol intoxication in the rat.

A study on possible modulating and direct effects of γ2-MSH and ACTH-(1–24) on the cardiovascular system of the rat

In conscious rats, γ2-melanocyte-stimulating hormone (γ2-MSH) dose-dependently increases blood pressure and heart rate, whereas adrenocorticotropin-(1–24) [ACTH-(1–24)] dose-dependently decreases blood pressure, an effect which was accompanied by a reflectory tachycardia. As the exact mechanism involved in these cardiovascular effects of the two melanocortins is as yet not known, we undertook a series of experiments to investigate the possibility that these peptides have modulating or direct effect on the cardiovascular system of the rat. In pithed rats γ2-MSH, administered intravenously (i.v.) in doses of 5–200 nmol/kg, had no significant effect on systolic and diastolic blood pressure and on heart rate, whereas ACTH-(1–24), 5–500 nmol/kg, i.v., dose-dependently decreased blood pressure and increased heart rate. Infusion of γ2-MSH, 10–8 M, or ACTH-(1–24), 10–6 M, in the isolated perfused rat heart did not significantly affect left ventricular pressure or coronary flow. Pretreatment with either γ2-MSH or ACTH-(1–24) did not modify the responsiveness of the myocardium and coronary vasculature to salbutamol and phenylephrine. Neither γ2-MSH nor ACTH-(1–24) did affect the vascular contractile machinery of skinned vascular smooth muscles of the rabbit with respect to Ca2+ handling in the cell, as measured by its sensitivity to exogenously applied Ca2+. γ2-MSH had no effect on blood pressure and heart rate in pithed rats in which postganglionic sympathetic outflow was stimulated by 1,1-dimethyl-4-phenylpiperazinium (DMPP), nor in pithed rats in which preganglionic sympathetic outflow was stimulated electrically. A dose of 15 nmol/kg ACTH-(1–24) had no significant influence on preganglionic outflow to the cardiac and vascular structures in pithed rats. These data show that γ2-MSH does not exert its cardiovascular effects via a peripheral site of action at the level of the vascular system and the heart, nor directly on pre- or postganglionic sympathetic outflow. These results are in support for the notion that the peptide acts via a brain region localised outside the blood-brain barrier. The acute depressor effect of ACTH-(1–24), however, seems to be due to a direct effect on the vasculature in the periphery.

Mepyramine but not cimetidine or clobenpropit blocks pertussis toxin-induced histamine sensitization in rats

The effects of pertussis toxin (PT) and the role of histaminergic H1, H2 and H3 receptor blockade on the actions of histamine on blood pressure, heart rate, blood gas values, and mortality were studied in anaesthetized rats. Four days after treatment with PT, histamine dose‐dependently decreased mean arterial blood pressure (MAP) and PT enhanced the histamine‐induced decrease in MAP. In the PT but not in the inactivated PT (IPT) or saline treated group three out of six animals died after the highest dose of histamine (300 mg kg−1, i.v.) In order to determine the type of histamine receptor that mediates HS, 4 days after PT the selective antagonists mepyramine (H1), cimetidine (H2) and clobenpropit (H3) were administered 20 min before the challenge with histamine. Mepyramine completely inhibited both the enhanced histamine‐induced decrease in MAP and mortality brought about by PT. Cimetidine and clobenpropit had no protective effects, but rather enhanced the histamine‐induced mortality elicited by PT. The present study shows that PT caused HS in rats which is primarily mediated via H1 and secondarily via H2 and H3 receptors. These results are considered to be a first step in the elucidation of the mechanism(s) of the HS test used in the quality control of acellular pertussis vaccine.

Cardiovascular responses to the stereoisomers of dobutamine in isolated rat hearts 48 hours after acute myocardial infarction.

Effects of acute myocardial infarction (48 h) oil cardiovascular responses to and dobutamme were studied in isolated perfused rat hearts. The effects of the racemate and the isomers on ventrienlat pressure were measured simultancolusly in infarctcd left ventricle and noninfarcted right ventricle. Administration of and dobutamine resulted in dose-dependent increase, in inotropic parameters and coronary flow (CF) in both control and in infareted hearts. As compared with the (-) isomer, the racemate and the (-)-isomer in both control and infarcted hearts were 1.5 and 15 times weaker with respect to inotropic parameter,. for and -dobutamine, there was no significant difference in pl)2 values in any of the motropic measurements between the infarcted group and the corresponding control group. The maximal obtainable response, were significantly lower in the infarcted group, as compared with their corresponding control group. In control hearts. effects of isoproterenol and methoxamine as compared with (-)-dobutamine were 2 log units more and 2 log units less in potency, respectively. The inotropic effects of (-)-dobutamine but not those of methoxamine were completely antagonized by propranolol (0.3 μM). Although the results provide evidence for the existence of myocardial α1-adrenoceptors. all forms of dohutamine everted positive isotropic effects through activation of β-adrenoceptors both in control and in infarcted rat hearts.

Modulation of antigen- and ischemia-induced effects by the platelet-activating factor antagonist WEB-2086 in isolated sensitized rat hearts

Summary: The modulatory role of the platelet-activating factor (PAF) antagonist WEB-2086 (30 µM) on the response to antigen-induced (trinitrophenyl-haptenized ovalbumin) and global ischemia (30 and 60 min)-induced changes in the response to antigen was studied in isolated hearts from actively sensitized rats. In sensitized normoxic hearts, both antigen (0.8 mg) and PAF (100 pmol) induced a short-term increase followed by a long-term decrease in coronary flow (CF). The antigen- but not the PAF-evoked increase in CF was accompanied by a substantial release of histamine. WEB-2086 enhanced the vasodilator effect and abolished the vasoconstrictor effect of 100 pmol of PAF but neither modified the coronary vascular effects of antigen nor the antigen-induced histamine release. Ischemia for 60 min followed by 30 min of reperfusion increased the diastolic left ventricular pressure but a 30-min period of ischemia and reperfusion had no effect on baseline cardiac function. WEB-2086 had no effect on ischemia-induced changes in cardiac function. A 30-min period of global ischemia enhanced the antigen-induced decrease in CF and systolic left ventricular pressure (SLVP). A 60 min period, however, suppressed the antigen-induced effects on CF and SLVP as well as antigen-induced histamine release. WEB-2086 partly protected the heart against the enhanced antigen-induced decrease in CF and SLVP after a 30-min period of global ischemia but no modulatory role of WEB-2086 was observed after 60 min of global ischemia. Our conclusions are that (a) PAF is not involved in rat cardiac anaphylaxis since WEB-2086 was proven to be inactive; (b) cardiac ischemia and cardiac anaphylaxis have interrelated mechanism of action since ischemia changed the anaphylactic response, which indicates that coincidence of these two pathological events could influence the clinical outcome; and (c) PAF is possibly involved in rat cardiac ischemia since WEB-2086 partly protected the heart against the enhanced antigen-induced decrease in CF and SLVP after 30 min of ischemia and reperfusion

Disturbance of cardiovascular circadian rhythms by pertussis vaccine in freely-moving rats.

Vaccination of young children with diphtheria, tetanus, poliomyelitis and pertussis (DTPoP) vaccine is effective in preventing outbreaks of whooping cough but adverse events sometimes occur. This pilot study shows that in freely-moving rats, multiple treatment with DTPoP (at day 0 and day 5, 6 ml/kg i.v.) increased heart rate (HR) for 5 days after the first treatment and decreased diastolic blood pressure (DBP) for at least 26 days after the first treatment and inhibited the circadian rhythm of HR and DBP for at least 10 days. DTPo vaccine, containing no pertussis vaccine, was free of such effects. Thus, in rats, the pertussis component of DTPoP acts on the cardiovascular system and disturbs its circadian rhythm. The contribution of these findings to clinical adverse effects is as yet unknown and needs further research.

Toxic doses of rac-, (−)-(S)- and (+)-(R)-propranolol in rats and rabbits

The contribution of the individual enantiomers ([+]-[R]- and [-]-[S]-propranolol) to rac-propranolol intoxication was studied in anaesthetized, spontaneously breathing (SB) rats and artificially ventilated (AV) rats and rabbits. In the SB rat, propranolol (30 mg.kg-1.h-1 i.v.) decreased heart rate and mean arterial blood pressure and caused hypoventilation, serious hypoxaemia, respiratory acidosis, and death by respiratory arrest. Survival time (ST) in the (+)-(R)-propranolol group (ST 91 +/- 5 min) was significantly longer than in the rac-propranolol group (ST. 68 +/- 6 min). In AV rats and rabbits toxic doses of rac-, (-)-(S)- and (+)-(R)-propranolol, 30 mg.kg-1.h-1 and 15 mg.kg-1.h-1 i.v., respectively, induced comparable effects on haemodynamic variables as in the SB rat. Artificial ventilation lengthened ST by a factor of three to four in rats. In the AV rat, STs were not significantly different between the rac-, (-)-(S)- and (+)-(R)-propranolol groups. In the rabbit, as in the SB rat, ST in the (+)-(R)-propranolol group was significantly longer than STs in the rac- and (-)-(S)-propranolol groups. The acute respiratory acidosis in SB rats and the prolonged ST in AV rats suggest that respiratory failure is the primary and cardiovascular failure the secondary cause of death in propranolol intoxication. The potentiation of the toxic effect of the enantiomers observed after dosing the racemate instead of the pure enantiomers could not be explained by a stereoselective difference in plasma propranolol concentration.