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

1. The transient release of 86Rb from parotid slices induced by secretagogues was investigated. 2. In the absence of external Ca, only one transient response (to carbachol) could be obtained. 3. After blocking the cholinergic stimulus with atropine, a second response (to substance P) could be elicited if the slices were briefly (2 min) exposed to a Ca-containing medium. 4. The magnitude of the substance P response depended on the concentration of Ca to which the slices had been exposed. 5. An exposure to Ca of 2 min duration was found to be sufficient to restore maximal substance P responsiveness. 6. These results are interpreted to mean that the cholinergic stimulus 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. 7. A number of cationic substances antagonized the restoration by Ca of the substance P response; the rank order of potency was: La3+ = Tm3+ greater than Co2+ = Ni2+ greater than neomycin much greater than Mg2+. 8. These same substances were examined for their relative abilities to inhibit Ca binding to phosphatidylinositol-4, 5-bisphosphate; in this case the rank order of potency was: La3+ = Tm3+ greater than neomycin greater than Co2+ greater than Ni2+ = Mg2+. 9. It is concluded that the uptake process does not appear to reflect Ca binding to phosphatidylinositol-4, 5-bisphosphate.