K. G. Morgan

Intracellular electrical activity of canine and human gastric smooth muscle.

1. Intracellular recordings were obtained from circular smooth muscle fibres of the canine fundus, corpus, antrum and pylorus as well as from the human corpus and antrum. 2. In the canine stomach, all regions of the stomach except the fundus exhibited spontaneous action potentials. 3. The spontaneous action potential consisted of an upstroke potential and a plateau potential. 4. There were regional differences in the configuration of the plateau potential. Corporal and antral smooth muscle did not normally spike during the plateau potential whereas terminal antral and pyloric muscle usually showed spikes on top of the plateau potential. Near the intermediate sphincter, there was a zone of transition in which oscillations in potential of variable amplitude were superimposed on the plateau potential. 5. The configuration of the action potential of the human stomach was similar to the configuration of the canine action potential when the same region of the stomach was compared. 6. The ionic dependence of the plateau potential was studied in canine stomach in an area where neither oscillations nor spikes occurred. 7. In clacium‐free solution, all spontaneous activity stopped. D600 selectively suppressed the size of the plateau potential. 8. Sodium‐deficient solution reduced the size of the plateau potential. 9. These results suggest that both calcium and sodium may be involved as current carriers in the generation of the plateau potential.

The inhibitory effects of vasoactive intestinal polypeptide on the mechanical and electrical activity of canine antral smooth muscle.

1. The inhibitory effects of vasoactive intestinal polypeptide (VIP) on the electrical and mechanical activity of canine antral smooth muscle were investigated. 2. In concentrations ranging from 5 X 10(‐9) to 1 X 10(‐7) M, VIP decreased the force of spontaneous contractions but had no measurable effect on spontaneous action potential complexes. 3. VIP had no effect on the increase in the amplitude and duration of the plateau potential and on the amplitude of contraction caused by a maximally effective concentration of acetylcholine. 4. VIP caused a significant decrease in the force of contraction caused by ED50 and threshold concentrations of acetylcholine. However, VIP had no measurable effect on the increase in the size of the action potential plateau caused by either concentration of acetylcholine. 5. VIP antagonized the increase in the amplitude of the plateau potential and the force of contraction induced by pentagastrin. It had no consistent effect on the pentagastrin‐induced increase in frequency. 6. The data indicate that VIP acts as an inhibitor in this tissue in two distinct ways. It uncouples electromechanical coupling during spontaneous and acetylcholine‐induced electrical and mechanical activity, and antagonizes pentagastrin‐induced increases in electrical and mechanical activities.

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.

The electrical basis for contraction and relaxation in canine fundal smooth muscle

1. Mechanical and intracellular electrical activities were recorded simultaneously from canine fundal and antral smooth muscle preparations.