TYROSINE PHOSPHORYLATION IN CONTRACTION OF OPOSSUM ESOPHAGEAL LONGITUDINAL MUSCLE IN RESPONSE TO SNP

Sodium nitroprusside (SNP) has been shown to elicit a guanosine 3',5'- cyclic monophosphate (cGMP)-mediated, indomethacin-sensitive contracti on of the opossum esophageal longitudinal muscle. We examined the role of tyrosine phosphorylation in the signal transduction pathway of con tractions induced by SNP and cGMP in longitudinal muscle strips in vit ro. Force of isometric contractions was expressed as the percentage of responses to KCl (73 mM). SNP (100 mu M)-induced contractions were 75 +/- 5% before and 3 +/- 2% after 50 mu M genistein (P < 0.005) and 86 +/- 16% before and 0 +/- 0% after 50 mu M tyrphostin B46. Contraction s in response to 8-bromo-cGMP (8-BrcGMP; 1 mM) were 74 +/- 15% before and 3 +/- 2% after genistein (P < 0.01) and 63 +/- 15% before and 18 /- 4% after tyrphostin B46 (P < 0.05). In contrast, KCl-induced contra ctions were 82 +/- 8% and 96 +/- 9% of the control value after geniste in and tyrphostin B46 treatments, respectively (P > 0.05 for both). Ca rbachol contractions were partially suppressed by genistein (106 +/- 8 % vs. 79 +/- 8%; P < 0.05) but unaffected by tyrphostin B46 (114 +/- 1 0% vs. 107 +/- 12%; P > 0.05). Western blot analysis revealed a 116-kD a phosphotyrosine protein in the control muscle strips. The level of t his protein was increased to 206 +/- 15% of control after SNP treatmen t. Both genistein and tyrphostin B46 blocked this increase. These stud ies show that contractions of the esophageal longitudinal muscle induc ed by SNP and cGMP utilize a signal transduction pathway different fro m that used by the depolarizing agent KCl and the muscarinic agonist c arbachol. Contractions induced by SNP and cGMP involve tyrosine phosph orylation of a protein, possibly identified as a 116-kDa protein, as a key step in the signaling pathway.