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

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Abstract
Mechanical and intracellular electrical activities were recorded simultaneously from canine fundal and antral smooth muscle preparations. Most fundal preparations displayed no spontaneous electrical or mechanical activity. The tissue had a space constant of 1.5 mm and a time constant of 189 ms and showed outward rectification in response to depolarizing current. Transmural nerve stimulation of fundal preparations demonstrated the presence of cholinergic excitatory and non-cholinergic, non-adrenergic inhibitory neural inputs to the tissue. The cholinergic nerve response consisted of a small, graded depolarization accompanied by a slow graded contraction; the inhibitory nerve response consisted of a graded hyperpolarization accompanied by a slow relaxation. The excitatory fundal nerve response was abolished or greatly diminished by D600 and Mn2+. D600 and Mn2+ also decreased basal tone. The inhibitory nerve response was unaffected by either agent. The excitatory nerve response in the fundus was contrasted with the excitatory nerve response in the antrum. In the fundus, stimulation of cholinergic motor nerves produced a depolarization which always produced a contraction. In the antrum, stimulation of cholinergic motor nerves between action potentials produced graded depolarizations of antral cells; there were no associated contractions. Stimulation of cholinergic motor nerves during spontaneous action potentials increased the amplitude and duration of the plateau phase of the action potential; this was associated with an augmentation of the spontaneous contractions. Voltage-tension curves were determined for antral and fundal preparations using K+ depolarization as a means of controlling membrane potential. Antral preparations displayed a voltage threshold for contraction at a membrane potential .apprx. 30 mV positive to the resting potential. Fundal resting potentials were at or more positive than their voltage thresholds. These differences in electromechanical coupling provide an explanation for the marked differences in the responses of fundal and antral smooth muscles to nerve stimulation and account for their physiologic function in vivo.