Effect of carbachol on radiosodium uptake by dispersed pancreatic acinar cells.

The effects of carbachol on uptake of 22Na by enzymatically dispersed rat pancreatic acinar cells were determined. Carbachol caused a slight but significant increase in uptake of 22Na by the cells in the presence of absence of ouabain (10(-3) M). A maximal response was obtained with 10(-6) M carbachol. The effects of carbachol were blocked by 10(-5) M atropine. Caerulein (10(-7) M) also stimulated 22Na uptake, while epinephrine (10(-4) M) and substance P (10(-7) M) did not. Carbachol did not stimulate 22Na uptake in the absence of extracellular Ca, although Ca omission significantly elevated basal 22Na uptake. The divalent cationophore A-23187 caused Ca-dependent 22Na uptake at 20 microM concentration but not at 0.3 microM. These results, when considered with earlier reports by others, suggest that muscarinic receptor activation leads to an increase in permeability of the acinar cell membrane to Na, and that Ca may be second messenger for this effect.

Binding of 125I-physalaemin to rat parotid acinar cells.

1. The binding of [125I]physalaemin to rat parotid acinar cells was investigated. 2. The [125I]physalaemin exhibited a small degree of specific binding that was rapid, reversible and saturable. 3. The EC50 values for inhibition of binding by four peptides were well correlated with their ability to activate Ca-dependent K release from the rat parotid gland. 4. The number of binding sites, which may represent substance P receptors, was estimated to be in the range of 200/cell, a value quite different from those reported previously for muscarinin (840/cell) or alpha-adrenergic (15,000/cell) receptors. 5. It is concluded that if, as previously suggested, these receptors regulate the same population of Ca channels, then the mechanism or perhaps efficiency by which this is achieved may differ for the three receptors.

The relationship between muscarinic receptor binding and ion movements in rat parotid cells.

1. The relationship of muscarinic receptor binding to ion fluxes induced by muscarinic agonists was investigated in rat parotid cells. 2. Receptor binding was measured with [3H]quinuclidinyl benzilate in living, dispersed parotid acinar cells. These same cells were used to determine concentration-effect relationships (ion fluxes) for agonists. 3. Receptor binding of antagonists accurately reflected the pharmacologically determined affinities. In the case of agonists, apparent pharmacological affinities were greater than binding affinities by a factor of 12.5 for methacholine and 18.7 for carbachol. 4. The reason for the discrepancy between agonist binding and effect is not known with certainty, but the existence of "spare" receptors is considered a possibility. 5. The total number of muscarinic receptors is estimated to be 23,000/cell. If some are spare, as few as 1800/cell may suffice to fully activate the available Ca channels.

Control of calcium channels by membrane receptors in the rat parotid gland.

1. The mechanism of action of agonists that stimulate K release from the parotid gland was investigated by monitoring efflux of 86Rb from rat parotid slices. 2. As in previous studies, the 86Rb release was increased by agonists in a biphasic manner. An early, transient phase occurred that was independent of extracellular Ca. This was followed by a sustained (or slowly falling phase) that required extracellular Ca. 3. The agonists were investigated as to their additivity and to their sensitivity to inhibition by a number of putative Ca-antagonists. 4. When carbachol, epinephrine and Substance P were employed in supramaximal concentrations, no combination of agonists produced a summated 86Rb release response, despite the fact that the Ca concentration was submaximal. 5. The sustained phase of 86Rb release, which is dependent on extracellular Ca, was blocked by La, Ni, Co and neomycin; the transient phase was unaffected by these agents. 6. The local anaesthetics tetracaine and procaine inhibited both the transient and sustained phases of the responses to carbachol and phenylephrine; responses to Substance P and to the divalent cationophore, A23187, were largely refractory to this effect. 7. These results support the contention that, in the parotid, muscarinic, alpha-adrenergic and peptide receptors regulate the same Ca influx sites. 8. Also, these results suggest that La, Ni, Co and neomycin appear to antagonize the action of Ca by impeding inward movement of the cation through activated Ca influx sites. 9. Finally, the local anesthetics appear to inhibit, and therefore, serve to define, a transduction step between receptor occupation and channel activation. 10. In the case of the Substance P mechanism, it appears that the transduction mechanism must be qualitatively different to that for the muscarinic or alpha-adrenergic mechanisms.