The Contraction of Smooth Muscle Cells of Intrapulmonary Arterioles Is Determined by the Frequency of Ca2+ Oscillations Induced by 5-HT and KCl

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Abstract
Increased resistance of the small blood vessels within the lungs is associated with pulmonary hypertension and results from a decrease in size induced by the contraction of their smooth muscle cells (SMCs). To study the mechanisms that regulate the contraction of intrapulmonary arteriole SMCs, the contractile and Ca2+ responses of the arteriole SMCs to 5-hydroxytrypamine (5-HT) and KCl were observed with phase-contrast and scanning confocal microscopy in thin lung slices cut from mouse lungs stiffened with agarose and gelatin. 5-HT induced a concentration-dependent contraction of the arterioles. Increasing concentrations of extracellular KCl induced transient contractions in the SMCs and a reduction in the arteriole luminal size. 5-HT induced oscillations in [Ca2+]i within the SMCs, and the frequency of these Ca2+ oscillations was dependent on the agonist concentration and correlated with the extent of sustained arteriole contraction. By contrast, KCl induced Ca2+ oscillations that occurred with low frequencies and were preceded by small, localized transient Ca2+ events. The 5-HT–induced Ca2+ oscillations and contractions occurred in the absence of extracellular Ca2+ and were resistant to Ni2+ and nifedipine but were abolished by caffeine. KCl-induced Ca2+ oscillations and contractions were abolished by the absence of extracellular Ca2+ and the presence of Ni2+, nifedipine, and caffeine. Arteriole contraction was induced or abolished by a 5-HT2–specific agonist or antagonist, respectively. These results indicate that 5-HT, acting via 5-HT2 receptors, induces arteriole contraction by initiating Ca2+ oscillations and that KCl induces contraction via Ca2+ transients resulting from the overfilling of internal Ca2+ stores. We hypothesize that the magnitude of the sustained intrapulmonary SMC contraction is determined by the frequency of Ca2+ oscillations and also by the relaxation rate of the SMC.