Studies of radiocalcium efflux in single barnacle muscle fibres: effects of procaine and external divalent cations

(45)Ca efflux from single barnacle muscle fibres loaded with radio-calcium by microinjection was studied.2. The (45)Ca washout curve consisted of three exponential phases with half-times of 4.8, 12.6 and 111.1 min.3. Removal of external Ca(2+) reduced (45)Ca efflux by 65%. The (45)Ca efflux recovered upon restoring external Ca(2+), the magnitude of the recovery being dependent upon the external Ca(2+) concentration. 10 mM procaine was found to reduce the magnitude of the recovery.4. Removal of external Mg(2+) resulted in a 38% increase in (45)Ca efflux.5. External application of procaine at pH 7.8 caused a dose-dependent inhibition of (45)Ca efflux. The magnitude of the inhibition was reduced in the presence of low external Ca(2+) concentrations. 10 mM procaine at pH 9.3 caused a biphasic effect: inhibition was followed by stimulation.6. Microinjection of 0.5 M procaine caused only inhibition of (45)Ca efflux, whereas microinjection of 1.5 M procaine caused stimulation followed by inhibition. These effects were observed at an external pH of 7.8 and 9.3.7. Injection of 100 mM-EGTA abolished the stimulatory but not the inhibitory effect produced by procaine injection.8. These results are interpreted as indicating that a major fraction of the (45)Ca efflux involves Ca-Ca exchange which is inhibited by the charged form of procaine in a non-competitive manner at the external surface of the muscle fibre. The stimulatory action is attributed to release by procaine of Ca(2+) from internal binding sites.