The Contribution of Calcium and Potassium to the Alpha-action of Adrenaline on Smooth Muscle Cells of the Portal Vein, Pulmonary Artery and Taenia Caeci of the Guinea-pig

The role of calcium and potassium in the alpha-action of adrenaline in pulmonary artery and portal vein was compared with that in taenia caeci by measuring changes in membrane potential, muscle contraction and ion fluxes in quiescent preparations from guinea-pigs (23 degrees C). The depolarization evoked by adrenaline (5 x 10(-8)-3 x 10(-5) M) was sustained in portal vein; in pulmonary artery it declined to a constant level after reaching an initial maximum. In calcium-free medium (20 min) containing EGTA (0.4 mM) and high magnesium (6.2 mM) adrenaline did not affect the membrane potential or the contractile state of the portal vein. Under these conditions the sustained phase of the response was abolished in the pulmonary artery; the remaining transient depolarization and contraction could be evoked only once. Adrenaline (3 x 10(-5) M) caused an increased 45Ca loss and 86Rb loss from the pulmonary artery and taenia caeci in calcium-free solution; a second addition of adrenaline to the calcium-free solution did not enhance the 45Ca loss from these tissues. The portal vein responded with an enhanced 86Rb loss on addition of the alpha-agonist. The bee toxin apamin (3 x 10(7) M) did not modify the depolarization, the contraction or the 45Ca and 86Rb fluxes evoked by adrenaline in the blood vessels. Enhancement of the 86Rb loss from taenia in the presence of adrenaline was prevented by apamin, but the excess loss of 45Ca was not abolished. It is concluded that adrenaline enhances cytoplasmic calcium by promoting calcium entry from the extracellular space in portal vein. In pulmonary artery and taenia caeci this is accompanied by mobilization of calcium from a cellular structure. Calcium entry facilitates triggering of the contractile proteins in vascular smooth muscle and is associated with membrane depolarization; in taenia caeci the mobilization of calcium caused by alpha-receptor activation is associated with the opening of potassium channels producing hyperpolarization and accordingly relaxation of the smooth muscle cells.