Regulation of potassium and magnesium effluxes by external magnesium in barnacle muscle fibres.

Fluxes and flux rate constants for potassium and magnesium were measured as a function of ambient magnesium concentration in giant muscle fibres from the acorn barnacle. Experiments were carried out in the absence of both external Na and Ca (to prevent possible effects of these ions), and fibres were depleted of internal Na and Ca by soaking in Na- and Ca-free solution before each experiment. K efflux was biphasic with respect to increases in external Mg concentration [( Mg+2]0). K efflux approximately doubled (from about 63 to 130 pmol/cm2.s) when [Mg+2]0 was increased from 2 to 5 mM; K efflux remained elevated in Mg up to 60 mM. However, at [Mg+2]0 = 120 mM, the stimulatory effect of external Mg vanished, and at 357 mM it was replaced by an inhibitory effect. Mg efflux was also biphasic with respect to [Mg+2]0, and this efflux was matched at low to moderate [Mg+2]0 by Mg influx of comparable magnitude. This study suggests the possible existence of a Mg transport system that can serve variously as a Na/Mg or a Mg/Mg exchanger that may require the participation of K ions for its operation.