Some further observations on the stimulation by high external potassium of the sodium efflux in barnacle muscle fibers.

A study has been made in single barnacle muscle fibers of the mechanism underlying the stimulatory response of the ouabain-insensitive Na efflux to high K0. Fibers poisoned with ouabain and preinjected with GTPNa2 show a biphasic stimulatory response to 100 mM K0 which is practically abolished by exposure of these fibers to verapamil. Prior or post-injection of cAMP-protein kinase type II regulatory subunits reduces the size of the response to 100 mM K0. Prior injection of cAMP-protein kinase catalytic subunits (CSU) attenuates the response to 100 mM K0 in fibers showing moderate sensitivity to CSU. In fibers showing extreme sensitivity to injected CSU, elevation of K0 partially reverses the sustained stimulatory response to CSU. Prior injection of Mg2+ leads to almost complete abolition of the response to 100 mM K0. External and internal application of F- reduces the response to 100 mM-K0. Injection of KF following decay of the response to high K0 results in a sustained stimulatory response. The drugs, chlorpromazine, trifluoperazine, imipramine and diphenylhydantoin are able to appreciably reduce the response to 100 mM K0. Taken together, these observations support the idea that the stimulatory response to high K0 is primarily due to activation by newly formed cAMP of cAMP-protein kinase, and that the transitory nature of the response and variable sensitivity of these fibers to high K0 is closely linked to the activity of a putative Mg2+-dependent protein phosphatase.