Contribution of both alpha- and beta-adrenoceptors to the inotropic effects of catecholamines in the rabbit heart

Effects of acetylcholine and noradrenalin on action potentials of isolated rabbit sinoatrial and atrial myocytes

The autonomic nervous system controls heart rate and contractility through sympathetic and parasympathetic inputs to the cardiac tissue, with acetylcholine (ACh) and noradrenalin (NA) as the chemical transmitters. In recent years, it has become clear that specific Regulators of G protein Signaling proteins (RGS proteins) suppress muscarinic sensitivity and parasympathetic tone, identifying RGS proteins as intriguing potential therapeutic targets. In the present study, we have identified the effects of 1 μM ACh and 1 μM NA on the intrinsic action potentials of sinoatrial (SA) nodal and atrial myocytes. Single cells were enzymatically isolated from the SA node or from the left atrium of rabbit hearts. Action potentials were recorded using the amphotericin-perforated patch-clamp technique in the absence and presence of ACh, NA, or a combination of both. In SA nodal myocytes, ACh increased cycle length and decreased diastolic depolarization rate, whereas NA decreased cycle length and increased diastolic depolarization rate. Both ACh and NA increased maximum upstroke velocity. Furthermore, ACh hyperpolarized the maximum diastolic potential. In atrial myocytes stimulated at 2 Hz, both ACh and NA hyperpolarized the maximum diastolic potential, increased the action potential amplitude, and increased the maximum upstroke velocity. Action potential duration at 50 and 90% repolarization was decreased by ACh, but increased by NA. The effects of both ACh and NA on action potential duration showed a dose dependence in the range of 1-1000 nM, while a clear-cut frequency dependence in the range of 1-4 Hz was absent. Intermediate results were obtained in the combined presence of ACh and NA in both SA nodal and atrial myocytes. Our data uncover the extent to which SA nodal and atrial action potentials are intrinsically dependent on ACh, NA, or a combination of both and may thus guide further experiments with RGS proteins.

Autonomic modulation of action potential and tension in guinea pig papillary muscles

The effects of alpha 1-adrenoceptor and muscarinic acetylcholine receptor stimulation on action potential and tension were studied in guinea pig papillary muscles obtained from both right and left ventricles. Stimulation of muscarinic acetylcholine receptors with carbachol produced a reduction of the action potential duration and a positive inotropic effect in papillary muscles from both ventricles. Both effects were concentration dependent and atropine sensitive. However, differential responsiveness was found upon alpha 1-adrenoceptor activation in muscles obtained from left and right ventricles. In right side papillary muscles, the alpha 1-adrenoceptor agonist, methoxamine, decreased the action potential duration and produced a positive inotropic effect. In contrast, methoxamine decreased the action potential duration but failed to produce a positive inotropic effect in left side papillary muscles. All methoxamine effects were antagonized by prazosin. Responses to maximum concentration of carbachol and methoxamine on the action potential duration and contractility were additive in right side papillary muscles. Phorbol 12,13-dibutyrate (PDB), a direct protein kinase C activator, also decreased the action potential duration in a manner that was additive to both carbachol and methoxamine. However, PDB reversed the positive inotropic effect of carbachol and methoxamine. The methoxamine-induced shortening of the action potential duration was prevented by pretreatment with indomethacin and nordihydroguaiaretic acid, blockers of arachidonic acid metabolism, but not by the protein kinase C antagonist, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7).(ABSTRACT TRUNCATED AT 250 WORDS)

Indirectly acting agonists. A model for the functional interaction of released endogenous double agonists

A theoretical model is presented to describe the functional interaction of an endogenous double agonist released by an indirectly acting agonist. The model takes into account the functional interaction of a released double (dualistic) agonist and describes both functional synergism and antagonism. It was shown that receptor density plays an important role in determining the profiles of concentration-effect curves and that it is necessary that the model should incorporate a parameter which describes receptor density. The model predicted that the shape of the concentration-effect curves may be sigmoid or bell-shaped. The theoretical predictions are comparable with experimental results.