Calcium dependence of neuromuscular events in esophageal smooth muscle of the opossum.

Intramural nerves in smooth muscle strips from opossum esophagus were stimulated electrically to cause contractions of longitudinal body muscle, off responses of transverse body muscle and relaxations of sphincter muscle. Krebs solution was modified by calcium removal by strontium substitution for calcium, by adding magnesium, and by adding nitroprusside. Longitudinal muscle contractions were abolished by calcium removal and by excess magnesium; they were unaffected by nitroprusside; strontium could not replace calcium. Off responses were abolished by calcium removal and by excess magnesium; they were depressed by nitroprusside; strontium effectively replaced calcium. Resting active tension in sphincter strips was partly reduced by calcium removal and by excess magnesium; it was abolished by nitroprusside; strontium could not replace calcium. Relaxations in sphincter strips were unaffected by all experimental conditions. Longitudinal contractions, off responses, and resting active tension of the sphincter represent different kinds of calcium activation of muscle. The excitatory nerves (which are cholinergic) in longitudinal muscle are calcium dependent, the nerves that produce off responses may not be, and the nerves that relax the sphincter are not.