Effects of metabolic inhibition on cytoplasmic calcium and contraction in smooth muscle of rat portal vein

Contractions in the rat portal vein, evoked by spontaneous action potentials or depolarizing high-K⁺ solution, are rapidly and reversibly inhibited by hypoxia or respiratory blockade. Intracellular free calcium ([Ca²⁺]_i) was measured using Fura-2 to evaluate the effects of metabolic blockade on excitation–contraction coupling. Spontaneous contractions were associated with transient increases in [Ca²⁺]_i. During exposure to cyanide (0.2–0.4 mm ) or 2,4-dinitrophenol (30 μm ) the duration and amplitude of the Ca²⁺ transients were decreased, leading to a decreased mean time integral of the individual [Ca²⁺]_i transient, and corresponding decrease in the duration and amplitude of the contraction. Basal [Ca²⁺]_i was increased in the presence of the metabolic inhibitors. High-K⁺ (40 mM) contractions caused a sustained increase in [Ca²⁺]_i, which was not inhibited by exposure to cyanide, although the amplitude of the associated contraction was greatly reduced. Together with the earlier demonstration of decreased 20 kD myosin light chain phosphorylation under these conditions, this indicates that the activation of contraction is influenced by metabolism via the energy dependence of the light chain phosphorylation reaction. Thus at least three steps in the excitation–contraction sequence are influenced by inhibition of oxidative metabolism: membrane excitation, light chain phosphorylation, and the cross-bridge cycle. This provides mechanisms for a high degree of metabolic sensitivity of vascular tone, of importance for the adaptation of blood flow to tissue metabolic demands.