Abstract
Direct effects of parathyroid hormone (PTH) on calcium uptake by isolated rat duodenal cell preparations enriched in enterocytes were investigated. PTH significantly stimulated enterocyte 45Ca2+ influx in a time-dependent (1-10 min) manner and at all doses tested (2 x 10(-13) to 10(-7) M). The Ca2+ channel antagonists verapamil (10 microM) and nitrendipine (1 microM) completely blocked the stimulation of Ca2+ influx by the hormone (10(-8) M). PTH markedly increased cAMP levels in rat duodenal cells (88, 167, and 67%, after 1, 2, and 3 min, respectively). In agreement with these observations, forskolin (adenylate cyclase activator), dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP), and Sp-cAMPS (cAMP analogs) mimicked, whereas Rp-cAMPS (cAMP antagonist) suppressed PTH and DBcAMP activation of enterocyte calcium uptake. Furthermore, the effects of DBcAMP were abolished by nitrendipine. These results show direct rapid effects of PTH on duodenal cells' Ca2+ influx, which involve the activation of a dihydropyridine-sensitive Ca2+ influx pathway and the cAMP second messenger system.
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