Modulation of rat parotid cell alpha-adrenergic responsiveness at a step subsequent to receptor activation

Parotid gland acinar cells, prepared from 12- and 24-month-old rats, show decreased physiological responsiveness to alpha-adrenergic stimulation in vitro compared to cells from 3-month-old rats. K+ efflux, an index of water and electrolyte secretion, was approximately 35% lower with 12- and 24-month-old parotid cells. No loss of alpha-adrenergic receptors, their binding affinity for specific alpha-adrenergic ligands, or their relative subtype distribution, accompanied the diminished exocrine function. Conversely, a significant reduction in alpha-adrenergic-mediated phospholipid turnover in, and 45Ca2+ efflux from, parotid cells of older rats was observed. These changes in phospholipid metabolism and Ca2+ flux were parallel to changes seen in K+ efflux as judged by dose-response studies. When the alpha-adrenergic receptor was by-passed by using the Ca2+-ionophore A-23187 to elicit K+ efflux, young and old parotid cells were equally responsive. In aggregate the findings suggest that parotid gland cells from older rats display an altered alpha-adrenergic signal transduction mechanism at a site between the receptor and phospholipid turnover/Ca2+ mobilization.