Agonist-dependent modulation of Ca²⁺ sensitivity in rabbit pulmonary artery smooth muscle

Abstract

The effects of the stable thromboxane analogue U46619, the α ₁-adrenergic agent phenylephrine and depolarization with high K⁺ on cytoplasmic Ca²⁺ ([Ca²⁺]_i) and force development were determined in rabbit pulmonary artery smooth muscle. Following stimulation with each of the excitatory agents, the time course of the [Ca²⁺]_i/force relationship described counter-clockwise hysteresis loops with the rise and fall in [Ca²⁺]_i leading, respectively, contraction and relaxation. The rank order of the force/[Ca²⁺]_i ratios evoked by the different methods of stimulation was: U46619 > phenylephrine high K⁺. The difference between the actions of U46619 and phenylephrine was due to the lesser Ca²⁺-releasing and greater Ca²⁺-sensitizing action of U46619. Both U46619 and phenylephrine also released intracellular Ca²⁺ in intact (non-permeabilized) preparations. The effects of the two agonists on force, at constant free cytoplasmic [Ca²⁺] maintained with EGTA, were also determined in preparations permeabilized with staphylococcal α-toxin, in which intracellularly stored Ca²⁺ was eliminated with A23187. Sensitization of the contractile response to Ca²⁺ by agonists was indicated by the contractile responses of permeabilized muscles to U46619 and to phenylephrine, in the presence of constant, highly buffered [Ca²⁺]_i. These contractions were inhibited by GDP[βS] and could also be elicited by GTP. We conclude that, in addition to changing [Ca²⁺]_i, pharmacomechanical coupling can also modulate contraction by altering the sensitivity of the regulatory/contractile apparatus of smooth muscle to [Ca²⁺]_i, through a G-protein-coupled mechanism.