Activation of alpha 2-adrenoceptors attenuates the inotropic effect of field stimulation in atria

Differential recruitment of alpha 1- and beta-adrenoceptors in inotropic control of atrial child myocardium by endogenous noradrenaline

Noradrenaline release, graded by frequency variation of field stimulation (0.1-2 Hz), in atrial myocardial specimens (n=45) from children (n=21) with congenital heart defects, was used to examine the inotropic responses of graded, receptor-selective, endogenous stimulation. Muscle trabeculae subjected to autonomic blockage by timolol, prazosin and atropine showed a slight positive force-frequency relationship (staircase phenomenon). Blockage by atropine/prazosin (i.e. beta-adrenoceptor stimulation) or atropine/timolol (i.e. alpha1-adrenoceptor stimulation) both resulted in positive inotropic effects. A group of specimens opposed by atropine and primarily subjected to frequency variation, secondly was returned to 1 Hz. Stabilization was followed by sequential reversal by beta-blocker (timolol), alpha 1-adrenoceptor stimulation by exogenous noradrenaline, reversal by alpha 1-blocker (prazosin), and finally supramaximal beta-adrenoceptor stimulation (isoprenaline). The maximal levels of inotropic responses mediated by exogenous alpha 1- and beta-adrenoceptor stimulation was estimated. Analysis of the contraction-relaxation cycles revealed that alpha1- and beta-adrenoceptors were recruited differentially. The alpha1-adrenoceptor mediated, endogenous inotropic effect at 1 Hz was close to the level obtained by exogenous noradrenaline stimulation. In contrast, less than 70% of the beta-adrenoceptor mediated, exogenous inotropic effect was expressed by endogenous noradrenaline at the same stimulating frequency, thus indicating that the alpha1-adrenoceptors may be located closer to the adrenergic nerve terminals than the beta-adrenoceptors. There may be a heterogeneous relationship within the same heart as to the relative distance between the nerve terminals and the adrenoceptors. Spatial localization of adrenergic receptors relative to adrenergic nerve terminals adds another aspect to adrenergic regulation. The alpha1-adrenoceptor pathway may play an important role, especially in low-intensity sympathetic inotropic myocardial control, whereas the beta-adrenoceptor pathway adds important effects to the high-intensity sympathetic regulation. Sympathetic activity may thus tonically stimulate the alpha1-adrenoceptor pathway, without necessarily stimulating the beta-adrenoceptor pathway to the same extent.

Myocardial alpha1-adrenoceptor: inotropic effect and physiologic and pathologic implications

Alpha1-adrenergic receptors have been found in myocardium of all mammalian species. Although the exact underlying mechanisms have not been conclusively determined, it would appear that the myocardial effects of alpha1-adrenoceptors may vary in importance according to the pathophysiologic process involved. In physiological conditions, this receptor system plays a role in cardiac growth, cardiac contraction, and has both an antiarrhythmic function as well as a role in cardiac adaptation to various situations. This system is also involved in some pathological processes such as ischemia/reperfusion, ischemic preconditioning, and cardiac hypertrophy. The role of alpha1-adrenoceptors in heart failure is somewhat controversial. Experimental evidence suggests that myocardial alpha1-adrenoceptors can have either beneficial or deleterious effects on the heart. It thus seems possible that the development of agents specific to certain subtypes of alpha1-adrenoceptor and a better understanding of their role in pathophysiologic states could be clinically relevant.

Quantification of the voltage-response relationship between punctate and field electrical stimulation and the function of isolated rat left atria and papillary muscles

The effects of varying the electrical stimulation voltage on the noradrenergic tone and systolic and diastolic cardiac function of isolated rat cardiac preparations is unknown. If a wide range of voltages substantially alters the basal noradrerengic tone of isolated cardiac preparations, highly variable responses may complicate the interpretation of the cardiac effects of interventions, such as drugs or toxins. This study was designed to determine whether field or punctate electrode stimulation altered systolic and diastolic cardiac function of isolated rat atrial and ventricular muscle preparations through a beta-adrenergic receptor-mediated mechanism. Isolated left atria and right ventricular papillary muscles from adult Sprague-Dawley rats were stimulated to contract isometrically in Krebs bicarbonate buffer (30 degrees C, pH 7.4, 0.5 Hz). Cardiac muscle function was assessed by determining cardiac contractility (dF/dt), relaxation (80% relaxation time), and muscle stiffness (changes in resting force). Muscles were electrically stimulated to contract using either punctate or field electrodes. Basal values of cardiac function were determined at threshold voltages, and voltage was increased in stepwise fashion, over a wide range, to obtain a voltage response-relation to cardiac function. In left atrial preparations, both punctate and field electrical stimulation caused a 200% increase in cardiac contractility. The greatest changes in contractility occurred at near threshold voltages (less than 5 volts for punctate and 19 volts for field stimulation). The voltage-dependent increases in cardiac contractility were attenuated or abolished by pretreatment with atenolol (10 micromol/L; selective beta 1-antagonist) or reserpine (5 mg/kg i.p., 24 hr before euthanasia). In contrast to left atrial preparations, neither field or punctate electrical stimulation had any effect on the cardiac contractility of papillary muscle preparations. Neither field or punctate electrical stimulation had any effect on cardiac relaxation (80% relaxation time) or muscle stiffness (changes in resting force) of left atrial or papillary muscle preparations. This study demonstrates that punctate and field electrical stimulation can significantly increase cardiac contractility of rat left atrial preparations at near threshold voltages by increasing noradrenergic tone. Thus, control of the voltage range may be required to reduce variation in cardiac function of isolated rat left atrial preparations.

Effects of presynaptic α-adrenoceptors and neuronal reuptake on noradrenaline overflow and cardiac response

Using an in situ perfused, innervated rat heart model, we studied the effects of presynaptic alpha-adrenergic and neuronal reuptake inhibition on evoked noradrenaline (NA) overflow and the postsynaptic response by sympathetic ganglion stimulation. NA overflow was significantly increased by neuronal reuptake inhibitors (desipramine and (+)-oxaprotiline) or by alpha-adrenoceptor antagonists (phentolamine and yohimbine), but the inotropic response was augmented only by alpha-antagonists. In the presence of desipramine, nerve stimulation induced a frequency-dependent increase in NA overflow and postsynaptic response, both were enhanced by yohimbine. In the absence of desipramine, however, postsynaptic response was potentiated by yohimbine despite an unchanged (at 2 and 5 Hz) or even reduced NA overflow (at 10 Hz). Thus, this study suggests that NA release and cardiac response are modulated by presynaptic alpha-adrenoceptors, and that the neuronal reuptake modifies the amount of NA overflow but has little effect on the postsynaptic response.

Pulse duration and alpha 2-adrenoceptors modify noradrenaline release from field-stimulated atria

In guinea-pig atria preloaded with 10 muCi [3H]noradrenaline, field stimulation during the refractory period increased the release of radioactivity and the force of contraction. Both effects were dependent on the extracellular Ca2+ concentration and were abolished by 3 x 10(-8) mol/l tetrodotoxin. When applied during each refractory period, two short (0.05 ms) pulses released significantly more radioactivity than one pulse of 0.1 ms duration applied during each refractory period. Similarly, a train of four pulses (0.05 ms each) was more effective than one pulse lasting 0.2 ms. The radioactivity released by single, long-lasting pulses in each refractory period was increased by phentolamine, idazoxan and N-ethylmaleimide. A small increase was also obtained with prazosin. The effect of phentolamine was antagonized by clonidine but not by phenylephrine. Clonidine did not prevent the effect of N-ethylmaleimide (all drugs 3 x 10(-5) mol/l; atropine 10(-7) mol/l and cocaine 3 x 10(-6) mol/l present in all experiments). It is concluded that activation of prejunctional alpha 2-adrenoceptors reduces the amount of noradrenaline release by one long field pulse in each refractory period. In most experiments, the inotropic effect of the two stimulation protocols was not significantly different. However, when atria were stimulated with four pulses immediately before stimulation with one pulse, the autoinhibition thus caused was strong enough to also reduce the inotropic effect of this stimulation protocol.