RECONSTITUTION OF SINGLE CALCIUM CHANNELS FROM SECRETORY GRANULES OF RAT ADENOHYPOPHYSIS IN PLANAR LIPID-MEMBRANES

Free cytosolic Ca2+ ([Ca2+]i) has been demonstrated to play a crucial role in the prolactin secretory pathway. It regulates events as apparently diverse as prolactin gene transcription and the fusion of secretory granules with the plasma membrane. It is therefore important to understand the mechanisms which regulate the level of [Ca2+]i. Because prolactin secretory granule membranes have been shown to contain large amounts of Ca2+ and there is evidence that this calcium can be released independently of the granule hormone content, we have investigated the possibility that prolactin secretory granule membranes contain Ca2+ channels. When purified prolactin secretory granules were fused with an artificial phospholipid bilayer, we found a Ca2+ channel with a linear current-voltage relationship (conductance approximately 45 pS) in symmetrical 50 mM CaCl2 solutions. Said channel had an open probability that was weakly dependent on the transmembrane potential, and a very good selectivity for calcium over chloride ions. The channel opened and closed very rapidly, when the majority of events lasting well below 25 ms. This channel could be important for the provision of high [Ca2+]i levels necessary for granule-plasma membrane fusion and could also be involved in the modulation of Ca2+ fluxes across the plasma membrane after the exocytotic release of prolactin.