Mechanisms Underlying Nitroglycerin-Induced Suppression of Phenylephrine- and Caffeine-Evoked Contractions of Smooth Muscles of the Rabbit Main Pulmonary Artery

Abstract

Using a strain measurement technique, we studied the mechanisms of the effect of a nitric oxide (NO) donor, nitroglycerin (NG), on contractions of smooth muscles of the main pulmonary artery of the rabbit induced by phenylephrine and caffeine in normal Krebs solution (NKS) or in nominally calcium-free solution (NCFS). Phenylephrine applications caused contractions consisting of an initial fast phasic low-amplitude component followed by a tonic higher-amplitude component. After caffeine-induced monophasic low-amplitude contraction, tension of the smooth muscle strip shifted below the conventional zero. Addition of NG to NKS resulted in a decrease in the smooth muscle tension below the conventional zero. Under the influence of NG, the initial phasic component of phenylephrine-induced contraction was partially suppressed, whereas the next tonic component was suppressed to a greater extent. At the same time, NG exerted nearly no influence on the amplitude of caffeine-induced contractions. Washing out by NKS of phenylephrine dissolved in NCFS resulted in initiation of a fast phasic high-amplitude contraction. Such a contraction did not develop either in the presence of NG or phenylephrine in NCFS or in the case of washing out of caffeine dissolved in NCFS. Our findings allow us to conclude that phenylephrine or caffeine added to the superfusate induce contractions of the smooth muscle cells (SMC) of the main pulmonary artery of the rabbit due to activation of Ca2+ release from the respective intracellular calcium stores. In addition, calcium ions entering SMC through the calcium channels of the plasma membrane are also involved in activation of the phenylephrine-induced contraction. The inhibitory effect of NG on the phenylephrine-induced contraction is related to the influence of NO on the release of Ca2+ from the inositol trisphosphate-sensitive intracellular calcium store and receptor-operated inflow of Ca2+ to SMC. Nitroglycerin did not significantly influence the caffeine-induced contraction and, therefore, Ca2+ release from the caffeine-sensitive store.