Effects of sodium nitroprusside and verapamil on lower esophageal sphincter

Infusions of verapamil (V) and sodium nitroprusside (NP) caused dose-dependent reductions in the lower esophageal sphincter pressure (LESP) in anesthetized opossums. Verapamil caused a gradual fall during the infusion and a gradual recovery upon cessation of the infusion. On the other hand, NP caused a prompt reduction and a prompt recovery. Both the agents also caused a fall in arterial blood pressure, but the fall in LESP was not related to the fall in the blood pressure. Both the agents antagonized the tonic and the phasic components of the LESP and, in large doses, both drugs almost abolished the LESP. Esophageal contractions were also antagonized by both agents, although verapamil was more effective than nitroprusside in this regard. These studies show that phasic and tonic components of LESP are not related to distinctive verapamil and nitroprusside sensitive calcium activation systems. Moreover, the V- and NP-sensitive calcium activation systems may lie in series rather than parallel.