Effects of enalapril on changes in cardiac output and organ vascular resistances induced by alpha 1- and alpha 2-adrenoceptor agonists in pithed normotensive rats

Cardiovascular effects of xylazine recorded with telemetry in the dog

Cardiovascular effects of xylazine have not been studied with telemetry in dogs. In the present study, the effects on cardiovascular parameters after intramuscular (i.m.) administration of 2.0 mg/kg xylazine were studied via telemetry in unrestrained dogs. Telemetry transmitters were implanted subcutaneously (s.c.) with a pressure catheter in the femoral artery. Cardiovascular effects and body temperature effects were assessed after i.m. administration of xylazine. Heart rate decreased for about 10 min and was continuously depressed during 60 min. Thereafter, heart rate slowly increased but had not fully reached pre-dose values 4 h after treatment. Both systolic and diastolic blood pressure increased immediately after administration of xylazine. The systolic blood pressure showed a peak increase for about 5-10 min and then decreased below the baseline value not normalizing within 90 min. The diastolic blood pressure peaked 5-10 min after xylazine administration but did not return to baseline level until 50 min after administration. Body temperature decreased continuously for about 90 min and remained low for more than 4 h after treatment. An additional administration of xylazine to the same individuals after a recovery period of 4 weeks induced exactly the same response in systolic and diastolic blood pressure and in heart rate. By using the telemetric recording system it was possible to continuously evaluate xylazine-induced cardiovascular responses in a way that is not possible with conventional techniques.

Interaction between alpha 2-adrenergic and angiotensin II systems in the control of glomerular hemodynamics as assessed by renal micropuncture in the rat

The hypothesis that renal alpha 2 adrenoceptors influence nephron filtration rate (SNGFR) via interaction with angiotensin II (AII) was tested by renal micropuncture. The physical determinants of SNGFR were assessed in adult male Munich Wistar rats 5-7 d after ipsilateral surgical renal denervation (DNX). DNX was performed to isolate inhibitory central and presynaptic alpha 2 adrenoceptors from end-organ receptors within the kidney. Two experimental protocols were employed: one to test whether prior AII receptor blockade with saralasin would alter the glomerular hemodynamic response to alpha 2 adrenoceptor stimulation with the selective agonist B-HT 933 under euvolemic conditions, and the other to test whether B-HT 933 would alter the response to exogenous AII under conditions of plasma volume expansion. In euvolemic rats, B-HT 933 caused SNGFR to decline as the result of a decrease in glomerular ultrafiltration coefficient (LpA), an effect that was blocked by saralasin. After plasma volume expansion, B-HT 933 showed no primary effect on LpA but heightened the response of arterial blood pressure, glomerular transcapillary pressure gradient, and LpA to AII. The parallel results of these converse experiments suggest a complementary interaction between renal alpha 2-adrenergic and AII systems in the control of LpA.

Mechanisms involved in angiotensin II induced increases in cardiac output in pithed rats

This study was conducted to determine the mechanisms by which angiotensin II (Ang-II) acutely increases cardiac output. Pithed Sprague-Dawley rats were prepared for continuous measurement of cardiac output by electromagnetic flowmetry. Ang-II (31-1000 ng/kg, i.v.) produced dose-related increases in cardiac output, heart rate and stroke volume. Although the heart rate increases were abolished by beta-adrenoceptor blockade, the cardiac output responses were unchanged due to an offsetting increase in stroke volume. The constancy of the cardiac output response following beta-adrenoceptor blockade suggested that Ang-II increased cardiac output by constricting venous smooth muscle and thereby increasing venous return. This conclusion is supported by the observation that Ang-II produced marked increases in left ventricular end diastolic pressure that paralleled the increases in cardiac output. In fact, based on volume loading with Tyrode's solution, the changes in left ventricular end diastolic pressure produced by Ang-II should have resulted in even greater increases in cardiac output. However, it appears that the significant rise in peripheral resistance to Ang-II tended to counter the effects of increased venous return on cardiac output. In addition, the Ang-II-induced elevations in cardiac output were not altered by alpha-adrenoceptor blockade. Therefore, catecholamines do not play a role in mediating the Ang-II effects. The results of this study support the conclusion that Ang-II is capable of increasing cardiac output by constriction of venous smooth muscle.

Effect of neuropeptide Y on cardiac output, its distribution, regional blood flow and organ vascular resistances in the pithed rat

1. The effects of neuropeptide Y on cardiac output, its distribution and organ vascular resistances were determined with tracer microspheres in pithed rats. 2. Neuropeptide Y increased blood pressure by increasing both cardiac output and total peripheral resistance. The increase in cardiac output was due to an increase in stroke volume as heart rate was not changed. Increased vascular resistance in the splenic, renal, testicular, epididymal, skeletal muscle, large intestinal and mesenteric vascular beds contributed to the increase in total peripheral resistance. Vasoconstriction was most pronounced in the mesenteric bed. 3. This study indicates that neuropeptide Y increases blood pressure by increasing cardiac output and total peripheral resistance. The increased cardiac output is possibly due to an increase in venous return, whilst the increased total peripheral resistance was due to regional vasoconstriction, particularly in the mesenteric bed.

Effects of moderate hypoxia, hypercapnia and acidosis on haemodynamic changes induced by endothelin-1 in the pithed rat

1. Pithed rats were respired at a fixed rate of 54 cycles min-1 and with a ventilation volume of either 20 (control) or 10 ml kg-1. In these two preparations, the dose-response relationships for the systemic blood pressure responses to endothelin-1, administered i.v., were examined. Also, cardiac output, its distribution, tissue blood flows and vascular resistances were determined at both respiratory volumes in pithed rats given saline or during pressor responses to endothelin-1 (750 ng, i.v.). Finally, a comparison was made of the pressor responses to endothelin-1 in the blood perfused superior mesenteric arterial bed of pithed rats respired at 10 or 20 ml kg-1. 2. In control rats the systemic blood pressure responses to i.v. endothelin-1 were biphasic with an initial, transient (30 s) decrease in blood pressure followed by a well sustained pressor response. These responses were dose-dependent (the ED50 for the pressor response being 0.27 +/- 0.04 micrograms). The pressor effect of endothelin-1 was due to an increase in total peripheral resistance with no change in heart rate or cardiac output. This increased total peripheral resistance was due to vasoconstriction of the spleen, stomach, large intestine, small intestine and the pancreas/mesentery (in which it was most severe). Endothelin-1 also increased blood flow through the heart, lungs, liver, epididimides, fat and skin through redistribution of cardiac output to these vascular beds. 3. At the lower ventilation volume there was moderate acidosis, hypoxia and hypercapnia relative to those rats respired at 20 ml kg-1. With respiration at 10 ml kg-1, the pressor response to endothelin-1 was not sustained and, after oscillations in both blood pressure and heart rate, death occurred 15-20 min after administration. The pressor effect resulted from increases in cardiac output (due to increased stroke volume) and total peripheral resistance: the latter was caused by vasoconstriction in the stomach, small intestine, large intestine and pancreas/mesentery. Endothelin-1 increased blood flow through the heart, lungs, liver, kidneys, testes, fat and skin due to either an increase in cardiac output, redistribution of cardiac output or both. 4. Endothelin-1 induced dose-dependent pressor responses in the mesenteric bed in situ. At the lower ventilation volume the potency of endothelin-1 in this vascular bed was increased approximately two fold with the ED50 being 68 +/- 7 pmol compared to 113 +/- 15 pmol in the rats respired at 20 ml kg-1. 5. This study indicates that, in normoxic control pithed rats, the pressor response to endothelin-1 was due largely to vasoconstriction of the splanchnic vascular bed. In rats with moderate hypoxia, hypercapnia and acidosis, the pressor response was due to vasoconstriction of the gastrointestinal tract as well as an increase in cardiac output. Endothelin-1 induced profound vasoconstriction in the mesenteric bed of the pithed rat both in vivo and in situ. The potency of endothelin-1 on this bed in situ was doubled by lowering the ventilation volume. An increase in cardiac contractility and severe gastrointestinal vasoconstriction may be the initial events leading to the eventual toxic effect of endothelin-1 in the hypoxic pithed rat.

Effect of a monoclonal antibody to angiotensin II on hemodynamic responses to noradrenergic stimulation in pithed rats

A specific angiotensin II monoclonal antibody, KAA8, was used to examine the interaction between sympathetic function and angiotensin II in pithed rats. KAA8, at 5 or 50 mg/kg i.v., did not alter the mean blood pressure, cardiac output, total peripheral resistance, or heart rate responses to sympathetic neural stimulation (0.25-4.0 Hz) or to norepinephrine (0.3-3 micrograms/kg i.v.) but blocked significantly the hemodynamic responses to angiotensin II (0.03-1.0 microgram/kg i.v.) and to angiotensin III (0.3-10 micrograms/kg i.v.). KAA8 treatment also reduced the plasma immunoreactive angiotensin II from 2,880 +/- 475 pg/ml to an undetectable level. In contrast, captopril (5 mg/kg i.v.) and saralasin (10 or 50 micrograms/kg/min i.v.) inhibited the mean blood pressure and total peripheral resistance responses, but not the cardiac output and heart rate responses, to sympathetic neural stimulation and to norepinephrine. These results, which confirm previous findings by Kaufman and Vollmer (Kaufman LJ, Vollmer RR: Endogenous angiotensin II facilitates sympathetically mediated hemodynamic responses in pithed rats. J Pharmacol Exp Ther 1985;235:128-134), demonstrate that angiotensin II selectively potentiates the sympathetic vascular function in the pithed rat. However, our results suggest that circulating angiotensin II does not appear to interact with the sympathetic vascular function. It is speculated that in the pithed rat the sympathetic vascular response is enhanced by vascular-formed angiotensin II.

Comparative effects of two antimycotic agents, ketoconazole and terbinafine on the metabolism of tolbutamide, ethinyloestradiol, cyclosporin and ethoxycoumarin by human liver microsomes in vitro

Two antimycotic agents, the azole ketoconazole and the allylamine terbinafine, have been examined for their effects on the metabolism of tolbutamide, ethinyloestradiol, cyclosporin and ethoxycoumarin by human liver microsomes (n = 4) in vitro. Ketoconazole caused marked inhibition of all enzyme activities with mean IC50 values (concentration producing 50% inhibition) of 17.9 microM (tolbutamide hydroxylase), 1.9 microM (ethinyloestradiol 2-hydroxylase), 2.0 microM (cyclosporin N-demethylase), 2.1 microM (cyclosporin hydroxylase) and 25 microM (ethoxycoumarin O-deethylase). At 50 microM terbinafine concentration, inhibition was less than 5% for tolbutamide and ethoxycoumarin, approximately 12% for both cyclosporin pathways and 35% for ethinyloestradiol. Terbinafine does not have the same inhibitory potential for cytochrome P-450 isozymes as ketoconazole.

Pressor effects of the alpha 2-adrenoceptor agonist B-HT 933 in anaesthetized and haemorrhagic rats: comparison with the haemodynamic effects of amidephrine

1. Blood pressure responses to single and multiple bolus doses of the alpha 2-adrenoceptor agonist B-HT 933 were analysed in intact anaesthetized rats which were either normotensive or hypotensive as a result of haemorrhage. Single bolus doses of B-HT 933 in normotensive rats induced a fall in blood pressure, whilst further doses induced dose-dependent pressor responses which were inhibited by the alpha 2-adrenoceptor antagonist yohimbine and unaffected by the alpha 1-adrenoceptor agonist prazosin. In the haemorrhagic, hypotensive animals, single bolus doses of B-HT 933 induced immediate dose-dependent pressor responses; the maximum pressor responses to the bolus of B-HT 933 and its ED50 values were the same in both the normotensive and hypotensive, haemorrhagic animals. 2. Cardiac output, its distribution and tissue blood flows were determined with tracer microspheres in intact anaesthetized normotensive and haemorrhagic, hypotensive rats during depressor (normotensive) and pressor (normotensive and hypotensive) responses to B-HT 933. Haemodynamics were also determined during pressor responses to the alpha 1-adrenoceptor agonist amidephrine. 3. In control normotensive rats, a single dose of B-HT 933 (1 mg kg-1) reduced blood pressure by reducing cardiac output (through a decrease in heart rate). It increased the fractional distribution of cardiac output to the spleen and stomach, reduced it to the heart and liver and reduced cardiac and hepatic blood flow. A further dose of B-HT 933 (1 mg kg-1 bolus followed by 100 micrograms min-1 infusion) increased blood pressure by increasing total peripheral resistance, which was accompanied by decreased proportions of cardiac output passing to the heart, liver and testes. There was also increased fractional distribution of cardiac output to the lungs, spleen, kidneys and stomach but blood flows through the liver and testes were reduced. Amidephrine (6 micrograms kg-1 bolus followed by 0.5 micrograms min-1 infusion) increased blood pressure by increasing cardiac output through an increased stroke volume. It increased cardiac output distribution to the kidneys and brain, increasing blood flow through the heart, lungs, brain, testes, epididimides, skin and large intestine. 4. Haemorrhage caused a fall in blood pressure which resulted from decreased total peripheral resistance and cardiac output (the latter due to decreases in both heart rate and stroke volume). It reduced the proportion of cardiac output distributed to the lungs, spleen, kidneys, testes and pancreas/mesentery and decreased blood flow through these organs as well as through the heart, liver, brain, epididimides, skin and the gastrointestinal tract.4