Modification of cardiac sodium current by intracellular application of cAMP

Abstract

We examined the effects of intracellular perfusion of cyclic adenosine monophosphate (cAMP) on the sodium current (I _Na) of guinea-pig ventricular myocytes, using the whole-cell clamp technique. I _Na was elicited by depolarizing voltage steps (−20 mV) from a variety of holding potentials (−120 to −50 mV), under conditions of 60 mM extracellular Na⁺ concentration ([Na⁺]₀) and at the temperature of 24–26°C.

Intracellular perfusion of cAMP decreased the I _Na elicited from the holding potentials less negative than −90 mV. In the presence of 1 mM cAMP, for example, the peak I _Na elicited from −80 mV decreased from 6.0±2.0 nA to 4.0±2.2 nA (mean±SD, P<0.02, n=7) within 3–6 min. In the presence of extracellular 3-isobutyl-1-methylxanthine (IBMX, 20 μM), much lower concentrations of cAMP (0.2 mM) yielded a comparable effect. On the other hand, intracellular perfusion of cAMP increased the I _Na elicited from very negative holding potentials (<−100 mV). For instance, the application of cAMP (1 mM) increased the I _Na elicited by step depolarizations from −120 mV (to −20 mV), from 9.9±2.1 nA to 11.0±3.1 nA (P<0.05, n=5).

The former effect was attributed to a marked shift of the steady-state inactivation curve of I _Na to the negative direction; the voltage of half-inactivation shifted from −77.9±1.0 to −83.5±1.4 mV, or by −5.6 mV. The latter effect may be explained by increases in maximum available conductance of I _Na. Extracellular application of isoproterenol (1 μM) also decreased the I _Na evoked from a holding potential of −80 mV, whereas it increased the I _Na elicited from more negative potentials of −120 mV. These effects of isoproterenol were reversible and markedly attenuated in the presence of a specific inhibitor of cAMP-dependent protein kinase, H-89 {N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulphonamide}, an isoquinolinesulphonamide derivative, in the extracellular medium (2–10 μM) and a protein kinase inhibitor (Walsh inhibitor) in the pipette solution (40 μM). H-89 (10 and 30 μM) affected neither the adenylate cyclase activity prepared from rabbit ventricular muscles, nor the isoproterenol-mediated increases in the cAMP content in guinea-pig ventricular muscles. Our observations suggest that the increase in intracellular cAMP modulates the function of cardiac Na channels, preferentially by stimulating cAMP-dependent protein kinase, with subsequent phosphorylation of the channel protein.