CA2-GLAND BASOLATERAL PLASMA-MEMBRANE VESICLES IS MODULATED BY MEMBRANE-POTENTIAL AND EXTRAVESICULAR [CA2+]( PERMEABILITY OF RAT PAROTID)

This study examines the Ca2+ permeability of basolateral plasma membra ne vesicles (BLMVs) isolated from the rat parotid gland by monitoring the rate of Ca-45(2+) efflux from actively-loaded (via the Ca2+-ATPase ) inside-out BLMVs. Ca2+ efflux from BLMVs into a K+-gluconate medium which hyperpolarizes the cytoplasmic side (i. e. outside) of the insid e-out BLMVs resulted in a faster rate of Ca2+ efflux compared with a c ontrol medium containing N-methyl-D-glucamine (NMDG)-gluconate. Conver sely, Ca2+ efflux into a medium which depolarizes the cytoplasmic side of the BLMVs (NMDG-chloride) resulted in slower rates of efflux compa red with those observed with the control medium. This increased rate o f Ca-45(2+) efflux from the hyperpolarized BLMV was inhibited by 1 mM Ni2+, yielding a rate of efflux similar to the rate observed in depola rized BLMVs. The rate of Ca2+ efflux from BLMVs was affected by [Ca2+] o ([Ca2+] on the extravesicular, cytoplasmic side of the vesicle). Whe n [Ca2+]o was kept > 200 nM during efflux, the rate of Ca2+ efflux fro m both hyper- and depolarized BLMVs was slow and relatively unresponsi ve to changes in [Ca2+]o, despite sizeable changes in the Ca2+ gradien t across the BLMV. However, when [Ca2+]o was lowered < 200 nM, there w as an abrupt increase in the rate of Ca2+ efflux from both hyper- and depolarized BLMVs. Additionally, when [Ca2+] was < 200 nM, the rate of Ca2+ efflux appeared to be more sensitive to driving force changes. T hese data suggest that Ca2+ permeability across the rat parotid gland basolateral plasma membrane is modulated by membrane potential and [Ca 2+] on the cytoplasmic side.