The action of catecholamines on guinea-pig taenia coli

Abstract

Observations of the effects of catecholamines on taenia coli indicate that the α and β actions are exerted on two different enzyme systems both involved in cellular mechanisms which regulate the intracellular Ca 2+ concentration, one by translocation of Ca within the cell and the other by removing Ca out of the cell. The β action reduces the tension response to m em brane excitation, probably by increased Ga uptake at intracellular stores, in all smooth muscles. The α effect is associated with a change in m em brane conductance resulting in hyperpolarization in some, including taenia coli, but depolarization in other smooth muscles. Nevertheless, the underlying mechanism may be the same, i.e. Ca release from intracellular stores and stimulation of Ca extrusion. This hypothesis is based on the assumption that the membrane permeability to K (and hence the membrane polarization and the degree of spontaneous activity) is largely determined by the amount of Ca bound at the inner surface of the plasma membrane. The change in membrane potential and membrane activity produced by the intracellular release of Ca caused by the α-adrenergic action will, therefore, depend on the ratio between the amount of Ca bound at the inside of the cell membrane and the rate of Ca pump activity in individual tissues.