The bimodal basis of the contractile response of the rabbit ear artery to norepinephrine and other agonists

Abstract

Strips of rabbit ear artery exhibit biphasic contractile responses to l-norepinephrine (1-NE), histamine, and serotonin. Evidence is presented on the basis of an analysis of the response to 1-NE that the two phases of contraction are associated with different modes of excitation and can be influenced independently. The initial part of the response is phasic. The response after 4 sec agonist exposure is the same as that after 32 sec. It is probably associated with local radial propagation of excitation in that the initial excitatory period is short, the contraction is preceded by electrical change and is abolished upon tissue exposure to a solution in which all NaCl and KCl is replaced by potassium methylsulfate. In contrast the second contractile phase is related to the time of tissue exposure to the drug, is an equilibrium-like response and is not dependent upon cell membrane polarization. Since this phase is more affected by calcium depletion than the first, the sources of activator calcium for the two phases may be different: that for the first phase many originate predominantly from intracellular and that for the second from extracellular sources. These findings support the hypothesis that the biphasic contractile response of the vessel reflects two different modes of excitation of the muscle wall by the agonists: the first, a triggered, propagated event; the second, a slow, equilibrium-type, non-propagated response.