Mechanisms of phasic and tonic actions of pentagastrin on canine gastric smooth muscle.

1. Pentagastrin increased the amplitude and frequency of spontaneous phasic contractions of gastric smooth muscle preparations from the antrum and corpus. Pentagastrin also increased the basal tone in the corpus but not in the antrum. 2. The increase in the amplitude of phasic contractions caused by pentagastri was associated with an increase in the amplitude and duration of the plateau of the action potential in both the antrum and corpus. 3. Voltage-tension curves generated by graded potassium depolariztion of the membrane potential defined a mechanical threshold for the muscle. 4. Comparison of the effects of pentagastrin to the voltage-tension curves indicated that the pentagastrin-induced depolarization of the resting potential was below threshold for the production of a contraction. In contrast, the movement of the top of the plateau potential to more depolarized potentials by this agent exactly followed the voltage-tension curve for the muscle, implying that both potassium and the plateau potential affected the same voltage-sensitive mechanism. 5. Pentagastrin caused an increase in conductance of antral preparations during the plateau of the action potential but not between action potentials. In corporal preparations, pentagastrin increased conductance during action potentials, but in addition increased conductance between action potentials. 6. The data indicate that pentagastrin increases the force of phasic contractions in antral and corporal smooth muscle by a voltage dependent process, probably involving the opening of voltage dependent calcium channels. In contrast, the data indicate that pentagastrin increases tone by a voltage independent process, possibly involving a nonregenerative increase in calcium movements through voltage independent channels.