The regulation of the calcium sensitivity of the contractile system in mammalian cardiac muscle.

Abstract

Treatment of rat ventricular cells with 10 mM EGTA [ethyleneglycol-bis(.beta.-aminoethyl ether)N,N,N'',N'' tetraacetic acid] makes the sarcolemma highly permeable to small ions and molecules without removing its restriction of the diffusion of larger molecules or inactivating all of its enzymatic functions. These hyperpermeable cardiac cells were used to study the regulation of the range of Ca concentration over which activation of the contractile proteins occurs (Ca sensitivity). Ca sensitivity can be varied from 3-6 fold without any significant alteration in the general shape of the relation between force and Ca concentrations. Although cyclic nucleotides in concentrations of 10-9-10-5 M do not influence Ca sensitivity, in the presence of a phosphodiesterase inhibitor, c[cyclic]GMP increases and cAMP decreases Ca sensitivity. Treatment of the hyperpermeable cells with a nonionic detergent raises Ca sensitivity, as does removal of the phosphate donor by complete substitution of CTP for ATP. Ca sensitivity is probably modulated by a cAMP-dependent phosphorylation that decreases Ca sensitivity. The sarcolemma is required for this reaction to take place. The effect of this reaction is antagonized by a cGMP-dependent reaction inside the cell. Studies involving the perfusion of the heart with and without epinephrine before the exposure to EGTA indicate that epinephrine can regulate this Ca sensitivity control system. The functional considerations of this regulatory system are discussed.