Stimulation of cyclic nucleotides of calcium efflux in barnacle muscle fibers.

Internal application of 10⁻⁴, 10⁻⁵, 10⁻⁶ and 10⁻⁷M cGMP and cAMP caused an increase in ⁴⁵Ca efflux in barnacle muscle fibers. Stimulation by either nucleotide occurred in the absence of external calcium and could be prevented by external application of 10 mM procaine or by prior internal treatment of these fibers with EGTA. The results indicate that cyclic nucleotides increase calcium efflux by releasing calcium from internal stores.