Differential functional properties of Ca stores in pulmonary arterial conduit and resistance myocytes

In smooth muscle cells, oscillations of intracellular Ca²⁺ concentration (Ca²⁺]_i) are controlled by inositol 1,4,5-trisphosphate (InsP₃) and ryanodine (Ry) receptors on the sarcoplasmic reticulum (SR). Here we show that these Ca²⁺ oscillations are regulated differentially by InsP₃ and Ry receptors in cells dispersed from the main trunk of the pulmonary artery (conduit myocytes) or from tertiary and quaternary arterial branches (resistance myocytes). Ry receptor antagonists inhibit either spontaneous or ATP-induced Ca²⁺ oscillations in resistance myocytes but they do not affect the oscillations in most conduit myocytes. In contrast, agents that inhibit InsP₃ production or activation of InsP₃ receptors do not alter the oscillations is resistance myocytes but block them in conduit myocytes. We have also examined the degree of overlap of Ry- and InsP₃-sensitive stores in myocytes along the pulmonary arterial tree. In conduit myocytes, depletion of Ry-sensitive stores with repeated application of caffeine in the presence of Ry or in Ca²⁺ free solutions did not prevent the ATP-induced Ca²⁺ release from InsP₃-dependent stores. However, responsiveness to ATP was completely abolished in resistance myocytes subjected to the same experimental protocol. Thus, InsP₃- and Ry-dependent stores appear to be separated in conduit myocytes but joined in resistance myocytes. These data demonstrate for the first time differential properties of intracellular Ca²⁺ stores and receptors in myocytes distributed along the pulmonary arterial tree and help to explain the distinct functional responses of large and small pulmonary vessels to vasoactive agents.