Negative inotropic effect of somatostatin in guinea-pig atrial fibres.

The effect of somatostatin (SS, 1 X 10(-7) M-5 X 10(-6) M) was studied on the electrical and mechanical properties of isolated atria of the guinea-pig. On spontaneously beating right atria, SS produced a dose-dependent negative inotropic effect which was accompanied by a decrease in atrial rate and a prolongation of the sinus node recovery time. In electrically driven left atria, SS produced a dose-dependent negative inotropic effect which occurred concomitantly with a decrease in the amplitude and duration of the plateau phase of the action potential of atrial fibres. SS also decreased the amplitude and maximum rate of depolarization of the slow action potential as well as the amplitude of the slow contractions induced by isoprenaline and caffeine in K-depolarized atrial fibres. The negative inotropic effect of SS varied with the concentration of Ca and Na in the bathing media and the frequency of stimulation. SS, 1 X 10(-6) M and 5 X 10(-6) M, decreased 45Ca uptake in electrically driven atria. All these results suggest that the negative inotropic effect produced by SS on rat isolated atria is related to its ability to reduce Ca influx via the slow inward Ca current.