CALCIUM-DEPENDENT REGULATION OF C1 SECRETION IN TRACHEAL EPITHELIUM

In primary cultures of dog tracheal epithelium, isoproterenol produced a transient increase in short-circuit current (I(sc)) (duration 30 s; maximal increase, 32 +/- 5-mu-A/cm2). This was followed by a more slowly developing sustained increase (9 +/- 3-mu-A/cm2), which mimicked the response to N6, 2'-O-dibutyryladenosine 3',5'-cyclic monophosphate (DBcAMP). The transient and sustained responses had dissociation constants for isoproterenol of 2 x 10(-8) and 2 x 10(-9) M, respectively. Bradykinin (in the presence of indomethacin), substance P, histamine, and thrombin produced only transient increases in I(sc). The time courses of these transients closely paralleled changes in concentration of intracellular Ca ([Ca2+]i) as measured with fura 2. For different mediators, there was a significant correlation between the maximal transient increase in I(sc) and the maximal increase in [Ca2+]i. The transients in I(sc) were not associated with elevation of adenosine 3',5'-cyclic monophosphate (cAMP) and were unaffected by pretreatment with DBcAMP, which abolishes the steady-state increase in response to isoproterenol. Both the transient increases in I(sc) and [Ca2+]i were inhibited by pretreatment with the Ca chelator 1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid. The phorbol ester 12-O-tetradecanoylphorbol 13-acetate abolished the transient increases in [Ca2+]i and I(sc) in response to isoproterenol but not to bradykinin. These results provide evidence that 1) isoproterenol and bradykinin elevate [Ca2+]i by different mechanisms, and 2) Ca elevation is associated with a transient increase in I(sc), whereas increased cAMP is associated with a smaller sustained increase.