Ca2+Permeability of Rat Parotid Gland Basolateral Plasma Membrane Vesicles is Modulated By Membrane Potential and Extravesicular [Ca2+]

Publisher Website
Google Scholar
Abstract
This study examines the Ca2+ permeability of basolateral plasma membrane vesicles (BLMVs) isolated from the rat parotid gland by monitoring the rate of45 Co2+ efflux from actively-loaded (via the Co2+-ATPase) inside-out BLMVs. Co2+ efflux from BLMVs into a K+-gluconate medium which hyperpolarizes the cytoplasmic side (i.e. outside) of the inside-out BLMVs resulted in a faster rate of Co2+ efflux compared with a control medium containing N-methyl-D-glucamine (NMDG)-gluconate. Conversely, Ca2+ efflux into a medium which depolarizes the cytoplasmic side of the BLMVs (NMDG-chloride) resulted in slower rates of efflux compared with those observed with the control medium. This increased rate of 45Ca2+ efflux from the hyperpolarized BLMV was inhibited by 1 mM Ni2+, yielding a rate of efflux similar to the rate observed in depolarized BLMVs. The rate of Ca2+ efflux from BLMVs was affected by [Ca2+], ([Ca2+] on the extravesicular, cytoplasmic side of the vesicle). When [Ca2+]o was kept > 200 nM during efflux, the rate of Ca2+ efflux from both hyper-and depolarized BLMVs was slow and relatively unresponsive to changes in [Ca2+]o, despite sizeable changes in the Ca2+ gradient across the BLMV. However, when [Ca2+]o was lowered < 200 nM, there was 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. These data suggest that Ca2+ permeability across the rat parotid gland basolateral plasma membrane is modulated by membrane potential and [Ca2+] on the cytoplasmic side.