Changes in membrane potential associated with cell swelling and regulatory volume decrease in barnacle muscle cells.

Our aim was to test the effect of hypotonicity and extracellular Ca2+ (Cao) on cell volume and membrane potential (VM) in barnacle muscle cells. Under isotonic conditions the resting VM of isolated cells mounted in the experimental chamber exposed to either Ca(2+)-free or Ca(2+)-containing (11 mM) solutions was -46.3 +/- 1.0 mV (n = 24) and -56.2 +/- 0.9 mV (n = 38), respectively. In the absence of Cao, the cells depolarized at a rate of 2.3 +/- 0.47 mV/hr; the presence of Cao reduced this rate of depolarization by 2.9-fold. Both in the absence or presence of Cao, the cells swelled in response to hypotonicity but underwent regulatory volume decrease (RVD) when Cao was present. Addition of the Ca2+ channel blocker, verapamil (0.1 mM), inhibited the Cao-dependent RVD. The percentage of cells responding with RVD increased with larger hypotonic challenges. There was a Cao-independent direct relationship between cell swelling and membrane depolarization which can be explained by dilution of the concentration of intracellular K+ ([K+]i). RVD was accompanied by a small hyperpolarization (3.0 +/- 0.38 mV/2 hr) which may represent increases in [K+]i during cell shrinking and activation of a conductive pathway. The results indicate the following: (1) the presence of Cao stabilizes VM; (2) cell swelling produces a depolarization which can be explained by dilution of [K+]i; (3) cell swelling activates a verapamil-sensitive Ca2+ influx responsible for promoting RVD; and (4) RVD is accompanied by a hyperpolarization which may result from activation of a conductive pathway.