Electrophysiological Studies on Spontaneous Activity of Vascular Smooth Muscle

Abstract

A classification of smooth muscle rhythms is derived from comparative studies in different smooth muscle preparations. On this basis, the spontaneous activity of the portal vein — as an example of a blood vessel with highly pronounced spontaneous activity — can be described as a superimposition of several rhythms: 1) minute-rhythm (MR), which has a frequency of 0. 5–2/min and is combined with only small fluctuations in the basic membrane potential; 2)basic organ specific rhythm (BOR) which divides the active phases of the MR and is connected with potential fluctuations of 10–30 mV; its frequency is 5–10 min; 3) faster oscillations of the membrane potential with period durations of ca. 1 sec (second-rhythm, SR); 4) spikes with amplitudes of between 40 and 60 mV, triggered by the SR and acting as a signal to trigger the mechanical tension development (GOLENHOFEN and v. LOH, 1970). Spikes and SR of portal vein are similar in mechanism to those of intestinal smooth muscle: They are, in the same way, Ca-dependent and are suppressed by iproveratril (GOLENHOFEN and LAMMEL, 1971). The MR is also similar in the different tissues. It therefore appears permissible to take smooth muscle of portal vein as the functional basis of vascular smooth muscle in general. Other types of vascular smooth muscle can be described as having developed from this basis by specialization and differentiation. The order of specialization can be seen — as a working hypothesis — roughly as follows: Portal vein — terminal venous vessels — terminal arterial resistance vessels — larger arteries — great elastic arteries. Myogenic automaticity is suppressed in this order, nervous control becomes stronger and more specialized and other processes appear in adaptation to special functions. The special properties of the action potential in elastic arteries can also be interpreted as part of a continuos system of vascular smooth muscle. As shown in fig. 1, the action potentials of elastic arteries are more similar in shape to the SR oscillations of portal vein and taenia coli than to the spike component of these tissues (the electrical activity of taenia coli in Ca-free solution, in fig. 1c, can be described as SR oscillations without the Ca-dependent spike component), and they, as the SR are more Na-dependent (KEATINGE, 1968).