Contractile serotonin-2A receptor signal transduction in guinea pig trachea: importance of protein kinase C and extracellular and intracellular calcium but not phosphoinositide hydrolysis.

Serotonin (5-HT) contracts the guinea pig trachea through stimulation of the 5-HT2A receptor, a receptor generally linked with phosphoinositide (PI) hydrolysis. However, previous limited evidence suggested that 5-HT did not increase PI hydrolysis in guinea pig trachea. The present studies confirmed that the 5-HT2A receptor is not coupled to PI hydrolysis and investigated the calcium source and involvement of protein kinase C (PKC) in 5-HT-induced contraction in guinea pig trachea. In vitro experiments, which used an enriched tracheal muscle preparation, confirmed the inability of 5-HT (10(-10) to 10(-2) M) to increase PI hydrolysis. Short incubations (1-60 min) of the trachea with 5-HT (10(-4) M) to minimize possible 5-HT2A receptor desensitization did not increase PI hydrolysis, whereas carbamylcholine (10(-7) to 10(-3) M) and histamine (10(-7) to 10(-4) M) did. These results demonstrate that, unlike most other 5-HT2A receptors, the 5-HT2A receptor in guinea pig trachea is not coupled to PI hydrolysis. The L-type calcium channel antagonists nitrendipine (10(-6) and 10(-5) M) and diltiazem (5 x 10(-5) M) significantly blocked maximal tracheal contraction to 5-HT (45-60%) inhibition) but not to carbamylcholine. The maximal response to 5-HT in calcium-free buffer (0 calcium, 0.05 mM EGTA) was also inhibited by 56%. The residual contraction to 5-HT in the absence of extracellular calcium suggested that at least a portion of the nitrendipine-insensitive 5-HT contraction was due to the release of intracellular calcium. In support of this idea, ryanodine (3 x 10(-5) M), a compound known to deplete intracellular calcium stores, depressed maximal 5-HT contraction in the presence of either nitrendipine or diltiazem. Neither calphostin C (4 x 10(-8) and 10(-6) M) or staurosporine (10(-8) M), both putative PKC inhibitors, affected tracheal contraction to 5-HT. However, the PKC inhibitor bisindolylmaleimide (5 x 10(-6) M), which abolished contraction to phorbol 12,13-dibutyrate (10(-6) M), unlike calphostin C, inhibited contraction to 5-HT in both the absence and presence of nitrendipine. This finding suggests that PKC activation is involved in 5-HT contraction. Thus, the tracheal 5-HT2A receptor is unique in that activation of the receptor does not result in PI hydrolysis but increases calcium influx through L-type voltage-dependent calcium channels, calcium release from the sarcoplasmic reticulum and activation of a bisindolylmaleimide-sensitive PKC.