Contrasting Action of H⁺ and Ca²⁺ on K⁺ Transport in the Diluting Segment of Frog Kidney

Abstract

In epithelia cellular K⁺ transport consists basically of K⁺ uptake via the Na⁺-K⁺ ATPase (pump) and K⁺ release via the K⁺ channel. To show whether pH and/or Ca²⁺ affect these two components of K⁺ transport, we applied the electrode sandwich technique on cells isolated from diluting segments of frog kidney. This experimental technique is suitable for studying transcellular K⁺ transport by measuring changes in extracellular K⁺ concentration with a K⁺-selective macroelectrode. Furthermore, K⁺ uptake can be separated from K⁺ release while extracellular pH and/or Ca²⁺ are changed. Na⁺-K⁺ pump and K⁺ channel respond to changes of extracellular pH in a parallel fashion. An increase of pH raises the activity of both transport systems. Both Na⁺-K⁺ ATPase and K⁺ channel exhibit pH optima, i.e., small changes in pH evoke pronounced K⁺ flux changes (pH 7.4–7.8). On the other hand, Ca²⁺ deprivation reduces K⁺ fluxes. The inhibition occurs over a wide pH range. The pump is more sensitive to Ca²⁺ deprivation than the channel. The experiments show that both Ca²⁺ and H⁺ take part in the regulation of K⁺ transport in a contrasting manner. Maximal activities of Na⁺-K⁺ ATPase and K⁺ channel are achieved with alkalosis in the presence of Ca²⁺. In contrast, K⁺ transport is reduced to a minimum with acidosis in the absence of Ca²⁺.