Physiological Localization of an Agonist-Sensitive Pool of Ca(2+) in Parotid Acinar Cells

摘要

Muscarinic stimulation of fluid secretion by mammalian salivary acinar cells is associated with a rise in the level of intracellular free calcium ((Ca2+)i) and activation of a calcium-sensitive potassium (K+) conductance in the basolateral membrane. To test in the intact cell whether the rise of (Ca2+)i precedes activation of the K+ conductance (as expected if Ca2+ is the intracellular messenger mediating this response), (Ca2+)i and membrane voltage were measured simultaneously in carbachol-stimulated rat parotid acinar cells by using fura-2 and an intracellular microelectrode. Unexpectedly, the cells hyperpolarize (indicating activation of the K+ conductance) before fura-2 detectable (Ca2+)i begins to rise. This occurs even in Ca2+-depleted medium where intracellular stores are the only source of mobilized Ca2+. Nevertheless, when the increase in (Ca2+)i was eliminated by loading cells with the Ca2+ chelator bis(2-amino-5-methylphenoxy)ethane-N,N,N',N'-tetraacetate (MeBAPTA) and stimulating in Ca2+-depleted medium, membrane hyperpolarization was also eliminated, indicating that a rise of (Ca2+) is required for the agonist-induced voltage response. We concluded that a receptor-mobilized intracellular store of Ca2+ is localized at or near the basolateral membrane. Reprints. (AW)