Positive inotropic effects of histamine in anaesthetized dogs

Changes in cardiovascular responsiveness to dopexamine and beta 1- and beta 2-adrenoceptor function after the chronic treatment of beta-adrenoceptor antagonists and agonists in anaesthetized dogs

1. The studies were designed to investigate the changes in responsiveness to beta-adrenoceptor agonists induced by chronic administration of beta-adrenoceptor antagonists and agonists. 2. Dose-response curves for dopexamine, isoprenaline, noradrenaline and impromidine on heart rate, blood pressure and myocardial contractility (dP/dt:integrated isometric tension) were obtained in untreated dogs and compared to those measured in dogs which had been pretreated with propranolol (8 mg kg-1 day-1), atenolol (6 mg kg-1 day-1), isoprenaline (0.5 microgram kg-1 min-1) or noradrenaline (0.5 microgram kg-1 min-1) for 14 days. 3. Propranolol pretreatment significantly enhanced the inotropic and chronotropic responses to isoprenaline. There were noticeable, but non-significant increases in inotropic sensitivity to noradrenaline and dopexamine, especially at higher dose levels. In the atenolol treatment group, there were significant increases in inotropic responses to dopexamine and isoprenaline and in depressor responses to isoprenaline. 4. Thus, chronic administration of propranolol increased responses mediated by both beta 1- and beta 2-adrenoceptors, whereas, atenolol selectively enhanced the inotropic responsiveness to dopexamine, which is mediated mainly by beta 2-adrenoceptors. 5. Isoprenaline pretreatment caused a significant decrease in inotropic sensitivity to dopexamine, isoprenaline and noradrenaline and a significant reduction in chronotropic responses to dopexamine. The tendency to reduced depressor responses to isoprenaline and dopexamine failed to reach significance. Pretreatment with noradrenaline decreased only the inotropic response to noradrenaline. 6. Thus, chronic isoprenaline treatment reduced the responsiveness of both beta 1- and beta 2-adrenoceptors, while chronic noradrenaline infusion only reduced beta 1-adrenoceptor-mediated responses. 7. There was no significant change in any of the dose-response curves to impromidine after any beta-adrenoceptor antagonist or agonist treatment. This indicates that there was no non-specific alteration in responsiveness and that the observed changes were likely to be associated with specific alterations in beta-adrenoceptor number or function.

Positive inotropic action of angiotensin II in the pithed rat

The cardiovascular effects of angiotensin II were examined in aortic blood pressure-controlled and -uncontrolled pithed rats. Angiotensin II induced a dose-dependent increase in diastolic blood pressure, left ventricular pressure (LVP), dP/dt (the first derivative of LVP) and heart rate in pithed rats. The maximal responses for these parameters were similar to those to noradrenaline, except for the rise in diastolic blood pressure, where noradrenaline caused a greater increase than angiotensin II. After treatment with propranolol, the positive chronotropic effect of angiotensin II was abolished. Angiotensin II produced a dose-dependent increase in diastolic blood pressure, which was similar to that of vasopressin, and an increase in dP/dtmax, which proved much greater than that of vasopressin. When aortic blood pressure was controlled and the beta-receptors were blocked by propranolol, angiotensin II caused a dose-dependent increase in dP/dtmax without affecting the left ventricular enddiastolic pressure. The same results were obtained after both beta- and alpha-adrenoceptors were blocked by propranolol and phentolamine. Losartan but not PD123177 caused parallel rightward shifts of the dose-response curve of angiotensin II for dP/dtmax in the aortic blood pressure controlled pithed rat without altering the maximal response. It is concluded that in the pithed rat angiotensin II produced an increase in myocardial contractile force which is not mediated by beta- or alpha-adrenoceptors. The inotropic effect appears to be mediated by angiotensin receptors, of the AT1-subtype.

The effects of neuropeptide Y on myocardial contractility and coronary blood flow

1. This study was designed to assess the effects of exogenous neuropeptide Y (NPY) on cardiac contractility and coronary blood flow (CBF) in anaesthetized dogs and to evaluate the effect of NPY on the responses to sympathetic and parasympathetic stimulation and to inotropic agents. 2. Bolus doses of NPY (500 micrograms), administered via the femoral vein, increased mean arterial pressure. The pressor effect was associated with reductions in heart rate, CBF and cardiac contractility. 3. The effects of NPY (500 micrograms) on contractility and CBF were compared with that of vasopressin (VP) (1 unit). For similar reductions in CBF, NPY and VP had similar negative inotropic effects. Thus, it is likely that the negative inotropic response to NPY is not due to a direct effect of NPY on the heart muscle but is largely due to coronary vasoconstriction. 4. In the presence of NPY (500 micrograms, i.v.), there was an inhibition of the positive inotropic response to stimulation of the left cardiac sympathetic nerve (2 or 5 Hz, 0.05 ms). NPY also inhibited the negative inotropic response and chronotropic response to vagal stimulation (2, 3 or 5 Hz, 0.05 ms). 5. Dose-response curves were obtained for the inotropic, chronotropic and pressor responses to cumulative infusions of noradrenaline (n = 6) and dobutamine (n = 6). NPY had no effect on these dose-response curves. 6. The effect of NPY on the responses to salbutamol and impromidine was assessed. NPY did not alter the positive inotropic, chronotropic or depressor responses to these agonists. 7. Our results indicate that NPY has direct, postsynaptic vasoconstrictor activity and the reduction in myocardial contractility and heart rate are due to a combination of coronary vasoconstriction, baroreceptor reflex activation and presynaptic inhibition of transmitter release from sympathetic nerves. The welldocumented inhibitory effect of NPY on the chronotropic response to parasympathetic stimulation was confirmed and similar inhibition of the inotropic response to both sympathetic and parasympathetic stimulation was demonstrated. Since there was no modulation of the inotropic effects of exogenous alpha- and beta-adrenoceptor or H2-receptor agonists, it is concluded that the effects of NPY on myocardial contractility are exerted presynaptically.

Local cardiac responses--alternative methods of control

Much attention has been paid to the influence of the beta-adrenoceptor system on cardiac function in heart failure. Full agonists and partial agonists acting on cardiac beta 1 receptors have been widely investigated, as has the density of these receptors in the failing heart. However, other cardiac control mechanisms may play important roles in the normal heart as well as in heart failure. The Frank-Starling mechanism of enhanced cardiac contraction produced by mechanical stretching of the ventricular myofibrils is well known. When treating patients with heart failure with diuretics, vasodilators and other drugs that influence preload, it is important to consider their overall effects in relation to the Starling curves. Atrial stretching also produces compensatory responses which are currently being intensively studied. Reflex release of atrial natriuretic factor after stimulation of atrial receptors has important physiologic effects in heart failure. The atria, but not the ventricles, are innervated by the vagus; the influence of the parasympathetic nervous system on the heart and circulation is often overlooked. The initial increase in heart rate during exercise is primarily due to withdrawal of vagal influence. Besides acetylcholine, the parasympathetic transmitter, many other local hormones may affect cardiac function; these include prostaglandins, 5-hydroxytryptamine and histamine. Although the activity of the sympathetic nervous system is mediated primarily through beta 1 adrenoceptors, both beta 2 and alpha receptors are also found in the heart. Myocardial alpha 1 receptors, which mediate a positive inotropic effect, have been identified, and prejunctional alpha 2 receptors may mediate inhibition of norepinephrine release from sympathetic nerves.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of histamine H2-receptor blockade on the cardiovascular reflex response to lower-body negative pressure in man

The role of histamine H2-receptors in cardiovascular control is unknown. In seven healthy volunteers, we studied how histamine H2-receptor blockade affected the reflex response to hypovolaemia induced by lower-body negative pressure (LBNP). In placebo-treated individuals, LBNP down to -40 mmHg did not change systolic pressure but increased diastolic pressure, heart rate, forearm vascular resistance, plasma noradrenaline concentration and plasma renin activity. After pretreatment with ranitidine, a specific histamine H2-receptor antagonist, the diastolic pressure rise no longer sufficed to maintain a constant systolic pressure during LBNP. Ranitidine pretreatment also attenuated the heart rate response and the rise in plasma renin activity induced by LBNP, but did not significantly change the reflex forearm vasoconstriction or the forearm plasma noradrenaline response. The results suggest that histamine H2-receptor blockade attenuates the reflex vasoconstrictor response to lower-body negative pressure. The mechanism behind this effect remains unknown, but the data do not support the idea that the effect is exerted in the skeletal muscle vascular bed.

Effects of histamine on atrial and ventricular contractility in the canine isovolumic heart

The effects of intracoronary administration of histamine on atrial and ventricular contractility were determined in a paced canine isovolumic heart preparation. Contractility was assessed by recording the pressure developed in saline-filled balloons placed in each of the four cardiac chambers. At doses above 0.1 mg and up to 100 mg histamine produced dose-related positive inotropic responses in all chambers. These were preceded by transient negative effects. The positive responses were not affected by a combination of H1 and H2 receptor antagonists antazoline and cimetidine but were almost completely abolished by the beta adrenoceptor blocker timolol. The negative responses were uninfluenced by either treatment. It was concluded that, in the canine isovolumic heart not subjected to complicating chronotropic and extracardiac factors, moderate doses of histamine are devoid of inotropic effects. Higher doses do produce myocardial stimulation, not mediated by histamine receptors, but probably due to norepinephrine release. These responses are preceded by transient non-specific depressant effects.