Ca2+ permeability of rat parotid gland basolateral plasma membrane vesicles is modulated by membrane potential and extravesicular [Ca2+].
This study examines the Ca2+ permeability of basolateral plasma membrane vesicles (BLMVs) isolated from the rat parotid gland by monitoring the rate of 45Ca2+ efflux from actively-loaded (via the Ca(2+)-ATPase) inside-out BLMVs. Ca2+ 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 Ca2+ 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+]o ([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.
Lockwich
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Clinical Investigations and Patient Care Branch, National Institute of Dental Research, National Institutes of Health, Bethesda, MD 20892.
Ambudkar
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IS
Shamoo
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AE
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002132:Calcium Radioisotopes; D002463:Cell Membrane Permeability / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D008297:Male; D008564:Membrane Potentials / Q000502:physiology*; D010306:Parotid Gland / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar
D002132:Calcium Radioisotopes; D002118:Calcium