pH regulation in barnacle muscle fibers: dependence on extracellular sodium and bicarbonate

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Abstract
Intracellular pH (pHi) regulation was studied in barnacle muscle fibers with pH-sensitive microelectrodes. The cells were acid loaded, and the subsequent recovery of pHi was monitored. The rate of recovery was reduced by 1/3 when external Na+ ([Na+]o) was replaced by Li+, but recovery was completely abolished when Na+ was replaced by choline or N-methyl-D-glucamine. In other experiments, varying amounts of Na+ were replaced by choline, and the acid extrusion rate, derived from the recovery rate of pHi, was calculated at a single value of pHi, 6.80. The dependence of the acid extrusion rate on [Na+]o could be described by Michaelis-Menten kinetics; at pHo (extracellular) = 8.0 and [HCO3-]o (extracellular) = 10 mM, the apparent Km and Vmax were 59 mM and 1.3 mmol.cntdot.l-1.cntdot.min-1. When [HCO3-]o was reduced to 2.5 mM at the same pHo, Km did not change significantly, but Vmax was substantially reduced. When pHo was reduced to 7.4 at constant [HCO3-]o, Vmax changed only slightly, but Km increased substantially. The dependence of the acid extrusion rate on [HCO3-]o also was examined. At pHo = 8.0 and [Na+]o = 440 mM, the apparent Km and Vmax were 4.1 mM and 2.1 mmol.cntdot.l-1.cntdot.min-1. When pHo was reduced to 7.4, Vmax was not altered, but Km substantially increased. The kinetic data are discussed in terms of the role of pHo, [Na+]o, and [HCO3-]o in the pHi-regulating system.