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

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.