5-HYDROXYTRYPTAMINE(2B) RECEPTOR SIGNALING IN RAT STOMACH FUNDUS - ROLE OF VOLTAGE-DEPENDENT CALCIUM CHANNELS, INTRACELLULAR CALCIUM-RELEASE AND PROTEIN-KINASE-C

The rat stomach fundus is enriched with the 5-hydroxytryptamine (5-HT) (2B) receptor, the newest subtype of the 5-HT2 receptor family to be c loned. Although the 5-HT2A and 5-HT2C receptor subtypes couple to phos phatidylinositol hydrolysis, such a coupling has not been established for the 5-HT2B receptor in tissues. Thus, the purpose of this study wa s to characterize further the signal transduction mechanism of the 5-H T2B receptor in rat stomach fundus. Nitrendipine (1 mu M) inhibited th e maximal contraction to 5-HT (1 mu M) by approximately 50%. Removal o f extracellular calcium did not inhibit 5-HT contraction to a greater extent than that produced by nitrendipine, indicating that calcium inf lux through voltage-dependent calcium channels was predominantly respo nsible for the dependence of the 5-HT contraction on extracellular cal cium. Depletion of both extracellular calcium and intracellular calciu m stores abolished 5-HT contraction. Ryanodine (30 mu M), a compound w hich inhibits calcium release from intracellular stores, significantly inhibited the maximal contraction to carbamylcholine (3 mu M). In con trast, ryanodine (30 mu M) did not inhibit the maximal contraction to 5-HT (1 mu M) in the absence of nitrendipine. However, ryanodine (30 m u M) did significantly inhibit the nitrendipine-insensitive 5-HT contr action, suggesting that this component of the contraction was due in p art to calcium release from a ryanodine-sensitive store. Bisindolylmal eimide (5 mu M), a specific inhibitor of protein kinase C (PKC), inhib ited 5-HT contraction in either the absence or presence of nitrendipin e, suggesting that activation of PKC is also involved. These data indi cate that the 5-HT2B receptor is coupled to calcium influx through vol tage-dependent calcium channels, intracellular calcium release and act ivation of PKC in the rat stomach fundus. Thus, the signal transductio n mechanisms coupled to the 5-HT2B receptor in rat stomach fundus may be unique relative to other members of the 5-HT2 receptor family.