Physiological role of electrogenic pumps in smooth muscle

Rhythmic electrical activity in the small intestine of cat appears to be initiated by small pacemaker potentials originating in the longitudinal muscle layer. These depolarizing potentials (slow waves) have durations of from 2 to 5 sec and are generally less than 15 mV in amplitude. Frequency depends on the region of intestine and varies between 3 and 10 waves per min. Electrical activity of the intact muscle coat is the summation of this pacemaker activity with triggered activity and is of larger amplitude and more complicated waveform. Exposure to ouabain (10(-5) M) or K-free saline causes a rapid block of pacemaker slow-wave activity and depolarization to levels more positive than the slow-wave crest. Repolarization does not restore spontaneity. Amplitude and frequency are relatively insensitive to changes in extracellular Na and Cl concentrations, and conductance changes, if they occur during the slow-wave cycle, are too small to be detected in the double-sucrose gap. Changes in extracellular Ca have pronounced effects on frequency, being decreased in low Ca and increased in high Ca. Calcium channel blockers have the same effects as low Ca saline, implying that the changes in frequency are mediated through internal Ca levels. Experiments using chlorotetracycline fluorescence provided data that intracellular Ca2+ levels oscillate even under conditions where measurable contractile activity is absent. We have proposed that oscillations in the activity of the Na-K pump are responsible for the pacemaker slow waves and that internal Ca2+ is an important cycle-to-cycle controlling factor in the oscillation.