PURINERGICALLY INDUCED MEMBRANE FLUIDIZATION IN CILIARY CELLS - CHARACTERIZATION AND CONTROL BY CALCIUM AND MEMBRANE-POTENTIAL

To examine the role of membrane dynamics in transmembrane signal trans duction, we studied changes in membrane fluidity in mucociliary tissue s from frog palate and esophagus epithelia stimulated by extracellular ATP. Micromolar concentrations of ATP induced strong changes in fluor escence polarization, possibly indicating membrane fluidization. This effect was dosage dependent, reaching a maximum at 10-mu M ATP. It was dependent on the presence of extracellular Ca2+ (or Mg2+), though it was insensitive to inhibitors of voltage-gated calcium channels. It wa s inhibited by thapsigargin and by ionomycin (at low extracellular Ca2 + concentration), both of which deplete Ca2+ stores. It was inhibited by the calcium-activated potassium channel inhibitors quinidine, chary bdotoxin, and apamine and was reduced considerably by replacement of e xtracellular Na+ with K+. Hyperpolarization, or depolarization, of the mucociliary membrane induced membrane fluidization. The degree of mem brane fluidization depended on the degree of hyperpolarization or depo larization of the ciliary membrane potential and was considerably lowe r than the effect induced by extracellular ATP. These results indicate that appreciable membrane fluidization induced by extracellular ATP d epends both on an increase in intracellular Ca2+, mainly from its inte rnal stores, and on hyperpolarization of the membrane. Calcium-depende nt potassium channels couple the two effects. In light of recent resul ts on the enhancement of ciliary beat frequency, it would appear that extracellular ATP-induced changes both in ciliary beat frequency and i n membrane fluidity are triggered by similar signal transduction pathw ays.