| Abstract: | The behavior of the ouabain-insensitive Na efflux in barnacle muscle fibers toward external high K and injection of Ca2+ has been further investigated. Raising Ke to 100 mM after the injection of 0.25 M or 0.1 M GTPNa2 results in a biphasic stimulatory response: the initial response is prompt in onset and small but transitory, whereas the delayed response is large and sustained. This second stimulatory phase is reduced markedly by injecting EGTA but not by PKI. Raising Ke to 100 mM in the presence of the 2 xanthine derivatives, viz. PMX and IAX, leads to a sustained stimulatory response of the ouabain-insensitive Na efflux which is halved by injecting PKI but unaffected by injecting EGTA. Injection of 0.1 M or 0.5 M CaCl2 in the presence of PMX and IAX leads to a sustained stimulatory response, which is almost completely abolished by injecting PKI but unaffected by injecting EGTA. These results confirm the earlier finding that the response of the ouabain-insensitive Na efflux to high Ke in fibers preinjected with GTPNa2 is biphasic and that the delayed second stimulatory phase is sustained rather than transitory. The ability of injected EGTA to only partially reverse the delayed response suggests that a fall in myoplasmic pCa is not the sole factor governing the kinetic picture. The experiments with PMX and IAX strongly suggest that cAMP is involved in the termination of the Ca2+ message. |
