Multiple Pacemaker Sites in Spontaneously Active Vascular Muscle

Multiple-electrode extracellular recording was used to determine the degree to which membrane electrical activity spreads through the rat anterior mesenteric-hepatic portal vein. Glass-pore electrodes filled with physiological salt solution were placed against the isolated intact vein or longitudinal strips of vein at 1-mm intervals to record spontaneous spiking. No pressure electrode effect was found in the rat portal vein. Patterns and amplitudes of spiking indicated that spike activity was conducted for only a few millimeters. Multiple pacemaker sites and decremental conduction were found in both normal physiological solution and solution in which calcium concentration was halved or doubled and potassium concentration was doubled or tripled. An average of three or four pacemaker sites was found in all solutions. Approximate equipotential contours were constructed from a grid map of average spike amplitudes, and they showed the multiple regions of high-amplitude spiking, the nonuniform spatial decay of spike amplitude, and the shift of high-amplitude sites with time.