NA-CA EXCHANGE IS REQUIRED FOR REST-DECAY BUT NOT FOR REST-POTENTIATION OF TWITCHES IN RABBIT AND RAT VENTRICULAR MYOCYTES

The influence of the Na-Ca exchange (NaCaX) on the effects of rest (30 -300 s) on twitch amplitude and SR Ca content (assessed by caffeine co ntractures) was studied in ventricular myocytes isolated from rat and rabbit. In control conditions, rabbit cells showed monotonic rest-deca y of the amplitudes of both twitch and caffeine contractures, while ra t myocytes developed rest-potentiation of twitches without change in S R Ca content. Inhibition of the Na-Ca exchange during rest by perfusio n with ONa,OCa solution did not affect the responses in rat cells but abolished rest-dependent SR Ca loss in rabbit cells. Indeed, when NaCa X was blocked during rest, then rabbit cells, like rat, displayed rest -potentiation of twitches. Stimulation of net Ca extrusion via NaCaX d uring rest by perfusion with OCa solution induced rest-decay of twitch es and caffeine contractures in rat cells similar to that observed in rabbit cells. This maneuver also accelerated decline in SR Ca during r est and amplitude of the first post-rest twitch in rabbit myocytes. Th ese effects were only slightly enhanced by preperfusion with ONa,OCa s olution to deplete Na-i. We were thus able to interconvert the contrac tile responses to rest between these cell types solely by modifying th e driving force for Ca transport by the exchange. Our results indicate that SR Ca is lost during quiescence in both species, but only if the NaCaX is able to promote diastolic Ca extrusion will net decline of S R Ca (and twitch amplitude) occur. On the other hand, post-rest twitch potentiation in both rat and rabbit cells can occur without a change in SR Ca content. This effects might be attributable, at least in part , to a slow phase of recovery of excitation-contraction coupling.