Agonist-induced vascular tone.

The cellular mechanisms underlying the agonist-induced sustained contraction of the vascular smooth muscle are reviewed in the light of the use of Ca2+ and the change of Ca2+ sensitivity of the contractile apparatus. It is generally accepted that the main trigger for contraction of vascular smooth muscle is the elevation of intracellular Ca2+ concentration. However, the measurement of intracellular Ca2+ concentration during the sustained phase of agonist-induced contraction is reported to be lower than that of high K+ stimulation or the value obtained by the experiments with chemically skinned smooth muscle preparations. These observations indicate that a second regulatory system may exist. One possible mechanism is the effectiveness of Ca2+ use. Agonist-induced Ca2+ influx may be more effective in raising the intracellular Ca2+ in the bulk of the cytoplasm than is Ca2+ entry induced by depolarization by the inhibition of a putative sarcoplasmic reticulum buffer barrier. Another possibility is the change of Ca2+ sensitivity of the contractile apparatus. Although the survey of the recent literature concerning the phorbol ester-induced vasoconstriction tends to support a role for protein kinase C in the change of Ca2+ sensitivity of the contractile proteins, it fails to establish a clear link between receptors, protein kinase C, and myofilaments. By using new methods for permeabilizing smooth muscle fibers, which retain the function of receptors and signal transduction systems, we now provide direct evidence that the activation of G protein by norepinephrine or guanosine 5'-0-(3-triphosphate) (GTP-gamma-S), nonhydrolyzable GTP analogue, enhances myofilament sensitivity to Ca2+.