Effects of uridine triphosphate on contractility, cyclic nucleotide levels and membrane potential in the isolated frog ventricle

Cyclic AMP phosphodiesterases and Ca2+ current regulation in cardiac cells

At least four different isoforms of phosphodiesterases (PDEs) are responsible for the hydrolysis of cAMP in cardiac cells. However, their distribution, localization and functional coupling to physiological effectors (such as ion channels, contractile proteins, etc.) vary significantly among various animal species and cardiac tissues. Because the activity of cardiac Ca2+ channels is strongly regulated by cAMP-dependent phosphorylation, Ca(2+)-channel current (ICa) measured in isolated cardiac myocytes may be used as a probe for studying cAMP metabolism. When the activity of adenylyl cyclase is bypassed by intracellular perfusion with submaximal concentrations of cAMP, effects of specific PDE inhibitors on ICa amplitude are mainly determined by their effects on PDE activity. This approach can be used to evaluate in vivo the functional coupling of various PDE isozymes to Ca2+ channels and their differential participation in the hormonal regulation of ICa and cardiac function. Combined with in vitro biochemical studies, such an experimental approach has permitted the discovery of hormonal inhibition of PDE activity in cardiac myocytes.

Effects of exogenous neurotransmitters on contractility and cyclic nucleotide metabolism in the isolated frog ventricle

A study has been made of the effects of acetylcholine and adrenaline on the metabolism of 3',5'-cyclic nucleotides by the isolated frog ventricle. Measurements of adenosine 3',5'-cyclic monophosphate (cyclic AMP) and guanosine 3',5'-cyclic monophosphate (cyclic GMP) were made: (a) at various times during responses produced by a given concn of acetylcholine (10(-7) M) or adrenaline (10(-6) M); and (b) at fixed times following exposure to different concns of the two agonists. The negative inotropic effect of acetylcholine is accompanied by reciprocal changes in the two cyclic nucleotides: cyclic GMP levels rise and cyclic AMP falls. Both effects are dose-related. A more complex pattern of changes occurs during responses produced by adrenaline. The initial increase in twitch tension is accompanied by a transient reduction in cyclic GMP and by a large increase in cyclic AMP. The rate at which cyclic AMP accumulates is greatest at the time when cyclic GMP levels are maximally depressed. An increase in cyclic GMP, seen later, coincides with a gradual fall in both twitch tension and cyclic AMP. The increments in both cyclic AMP and cyclic GMP, measured when the twitch is maximally potentiated, are dose-related. The magnitude of the decrease in peak twitch tension produced by acetylcholine and of the increase produced by adrenaline correlate closely with corresponding changes in the relative proportion of cyclic AMP: cyclic GMP present in the fibres. The possible significance of the changes in cyclic AMP and cyclic GMP in modulating the response of the ventricle is discussed.

Stretch stimulates cyclic nucleotide metabolism in the isolated frog ventricle

An investigation has been made of the effect of stretch on cyclic nucleotide metabolism. The levels of endogenous adenosine 3',5' - cyclic monophosphate (cyclic AMP) and guanosine 3',5' - cyclic monophosphate (cyclic GMP) were measured during different patterns of stretch and release. Stretch to Lmax produced a moderate increase in cyclic AMP and a proportionately greater increase in cyclic GMP; as a result, the ratio cyclic AMP: cyclic GMP fell to around 0.5 x control (unstretched) value. The effects of releasing previously stretched preparations resulted in a decrease in cyclic GMP levels. In contrast, the increase in cyclic AMP level following stretch was not affected by this procedure. Nevertheless, these effects together produced an increase in the ratio cyclic AMP: cyclic GMP, the magnitude of which increased linearly with the amplitude of release.