A constitutively active nonselective cation conductance underlies resting Ca2+ influx and secretion in bovine adrenal chromaffin cells

We have combined fluorimetric measurements of the intracellular free Ca2+ concentration ([Ca2+]i) with the patch clamp technique, to investigate resting Ca2+ entry in bovine adrenal chromaffin cells. Perfusion with nominally Ca2+-free medium resulted in a rapid, reversible decrease in [Ca2+]i, indicating a resting Ca2+ permeability across the plasma membrane. Simultaneous whole-cell voltage-clamp showed a resting inward current that increased when extracellular Ca2+ (Ca2+o) was lowered. This current had a reversal potential of around 0 mV and was carried by monovalent or divalent cations. In Na+-free extracellular medium there was a reduction in current amplitude upon removal of Ca2+o, indicating the current can carry Ca2+. The current was constitutively active and not enhanced by agents that promote Ca2+-store depletion such as thapsigargin. Extracellular La3+ abolished the resting current, reduced resting [Ca2+]i and inhibited basal secretion. Abolishment of resting Ca2+ influx depleted the inositol 1,4,5-trisphosphate-sensitive Ca2+ store without affecting the caffeine-sensitive Ca2+ store. The results indicate the presence of a constitutively active nonselective cation conductance, permeable to both monovalent and divalent cations, that can regulate [Ca2+]i, the repletion state of the intracellular Ca2+ store and the secretory response in resting cells.