Effect of calcium and cAMP on Go alpha expression in neonatal rat cardiac myocytes

Culturing neonatal rat cardiac myocytes in 50 mM KCl inhibits the accumulation of Go alpha that occurs when myocytes are placed in culture. The mechanism by which high extracellular K+ inhibits Go alpha accumulation in myocytes was investigated by measurement of the concentration of intracellular Ca2+ ([Ca2+]) and adenosine 3',5'-cyclic monophosphate concentration ([cAMP]) of control and K(+)-depolarized myocytes. Although intracellular [Ca2+] in K(+)-depolarized myocytes was twofold higher than basal intracellular [Ca2+] in control cells, the mean intracellular [Ca2+] in contracting control myocytes was comparable to that of K(+)-depolarized myocytes. Furthermore, myocytes cultured in low Ca2+ plus high K+ exhibited an inhibition of Go alpha accumulation, even though intracellular [Ca2+] was 10-fold lower than that of cells cultured in normal Ca2+ plus high K+. In addition, intracellular [cAMP] of K(+)-depolarized myocytes was comparable to that of control cells. Moreover, dibutyryl cAMP inhibited Go alpha accumulation in myocytes to the same extent as high K+, even though intracellular [cAMP] differed 10-fold. Thus neither intracellular Ca2+ nor cAMP appear to mediate the inhibitory effect of high K+ on Go alpha accumulation. However, cAMP has an inhibitory effect on Go alpha expression that is independent of K+.