HISTAMINE-INDUCED CA2+ ENTRY PRECEDES CA2+ MOBILIZATION IN BOVINE ADRENAL CHROMAFFIN CELLS

The relationship between histamine-induced Ca2+ mobilization and Ca2entry in bovine adrenal chromaffin cells has been investigated. Stoppe d-flow fluorimetry of fura-2-loaded chromaffin cell populations reveal ed that 10 mu M histamine promoted entry of Ca2+ or Mn2+ without measu rable delay (less than or equal to 20 ms), through a pathway that was insensitive to the dihydropyridine antagonist nifedipine. In the absen ce of extracellular Ca2+, or in the presence of 100 mu M La3+, a block er of receptor-mediated Ca2+ entry, 10 mu M histamine triggered an ele vation in intracellular calcium concentration ([Ca2+](i)), but only af ter a delay of approx. 200 ms, which presumably represented the time r equired to mobilize intracellular Ca2+. These data suggested that hist amine-induced bivalent-cation entry precedes extensive Ca2+ mobilizati on in chromaffin cells. In order to confirm that histamine can promote Ca2+ entry largely independently of mobilizing intracellular Ca2+, th e ability of histamine to promote Ca2+ entry into cells whose intracel lular Ca2+ store had been largely depleted was assessed. Fura-2-loaded chromaffin cells were treated with 10 mu M ryanodine together with 40 mM caffeine, to deplete the hormone-sensitive Ca2+ store. This result ed in an approx. 95 % inhibition of histamine-induced Ca2+ release. Un der these conditions, histamine was still able to promote an entry of Ca2+ that was essentially indistinguishable from that promoted in cont rol cells. In single cells, introduction of heparin (100 mg/ml), but n ot de-N-sulphated heparin (100 mg/ml), abolished the histamine-induced rise in [Ca2+](i). All these data suggest that histamine can induce G -protein- or inositol phosphate-dependent rapid (less than or equal to 20 ms) Ca2+ entry without an extensive intracellular mobilization res ponse in chromaffin cells, which points to activation of an entry mech anism distinct from the Ca2+-release-activated Ca2+ channel found in n on-excitable cells.