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 contraction of the opossum esophageal longitudinal muscle. We examined the role of tyrosine phosphorylation in the signal transduction pathway of contractions induced by SNP and cGMP in longitudinal muscle strips in vitro. Force of isometric contractions was expressed as the percentage of responses to KCl (73 mM). SNP (100 μM)-induced contractions were 75 ± 5% before and 3 ± 2% after 50 μM genistein (P < 0.005) and 86 ± 16% before and 0 ± 0% after 50 μM tyrphostin B46. Contractions 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 contractions were 82 ± 8% and 96 ± 9% of the control value after genistein and tyrphostin B46 treatments, respectively (P > 0.05 for both). Carbachol contractions were partially suppressed by genistein (106 ± 8% vs. 79 ± 8%; P < 0.05) but unaffected by tyrphostin B46 (114 ± 10% vs. 107 ± 12%; P > 0.05). Western blot analysis revealed a 116-kDa phosphotyrosine protein in the control muscle strips. The level of this protein was increased to 206 ± 15% of control after SNP treatment. Both genistein and tyrphostin B46 blocked this increase. These studies show that contractions of the esophageal longitudinal muscle induced by SNP and cGMP utilize a signal transduction pathway different from that used by the depolarizing agent KCl and the muscarinic agonist carbachol. Contractions induced by SNP and cGMP involve tyrosine phosphorylation of a protein, possibly identified as a 116- kDa protein, as a key step in the signaling pathway.