INTRINSIC REGULATION OF K+ TRANSPORT IN MALPIGHIAN TUBULES (FORMICA) - ELECTROPHYSIOLOGICAL EVIDENCE

Abstract

Intracellular basolateral (V(bl)) and, indirectly, apical membrane potentials (V(ap)) have been measured in spontaneously secreting isolated Malpighian tubules of Formica. V(bl) was sensitive to the bath K+ concentration, [K+]bl, (42 mV/decade) and very little to the Na+ and Cl- concentration. V(ap) was also sensitive to [K+]bl. The voltage changes in V(ap) were 60-70% of those in V(bl), so the transepithelial potential (V(te)) also changed: it increased with [K+]bl (lumen became more positive). The overall result of an increase in bath K+ concentration was a facilitation of KCl secretion: the apical electrical gradient to be overcome for K+ extrusion was lowered and the transepithelial electrical gradient favouring Cl- movement to the lumen was increased. Transient changes in V(bl) on varying [K+]bl also suggested dependence of the cellular K+ concentration on [K+]bl. When plotting fluid secretion rate, i.e. K+ transport, as a function of the calculated apical electrochemical gradient for K+, a highly significant inverse relationship was found. The gradient itself was very closely correlated with [K+]bl. In another series of experiments tubules were luminally perfused with symmetrical solutions (51 mM K+ 143 mM Cl-) and resistances were measured after current injection. We report a transepithelial length-specific resistance of 23 +/- 3 k-OMEGA-cm, a tissue specific resistance of 182 +/- OMEGA-cm2 and a length constant of 402 +/- 29-mu-m (n = 38). From intracellular measurements and luminal injection of current the voltage divider ratio could be obtained. This gave an estimate of the ratio of apical over basolateral resistance of 44 +/- 8 (n = 6). Analysis of the electrical equivalent model. taking into account the high apical over basolateral resistance, explained why changes in the basolateral electromotive forces were reflected on the apical membrane. Taken together the results of this study were consistent with a role for the high basolateral conductance (relative to the apical conductance) in the intrinsic regulation of K+ transport by bath [K+]. A second mechanism of regulation could be the fact that [K+]bl determines the amount of cellular K+ available for transport.