Nature of the receptor‐regulated calcium pool in the rat parotid gland.

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Abstract
The transient release of 86Rb from parotid slices induced by secretagogues was investigated. In the absence of external Ca, only 1 transient response (to carbachol) could be obtained. After blocking the cholinergic stimulus with atropine, a 2nd response (to substance P) could be elicited if the slices were briefly (2 min) exposed to a Ca-containing medium. The magnitude of the substance P response depended on the concentration of Ca to which the slices had been exposed. An exposure to Ca of 2 min duration was sufficient to restore maximal substance P responsiveness. The cholinergic stimulus apparently elicited a transient 86Rb efflux response by first releasing a finite pool of cellular Ca which could be reloaded from the extracellular space by a brief (2 min) incubation in a Ca-containing medium. The magnitude of the subsequent response to substance P apparently reflects the quantity of Ca taken up by the pool. A number of cationic substances antagonized the restoration by Ca of the substance P response; the rank order of potency was: La3+ = Tm3+ > Co2+ = Ni2+ > neomycin .mchgt. Mg2+. These same substances were examined for their relative abilities to inhibit Ca binding to phosphatidylinositol-4,5-biophosphate; in this case the rank order of potency was: La3+ = Tm3+ > neomycin > Co2+ > Ni2+ = Mg2+. The uptake process does not appear to reflect Ca binding to phosphatidylinositol-4,5-bisphosphate.