Involvement of P2-purinergic receptors in intracellular Ca2+ responses and the contraction of mammary myoepithelial cells.

Mammary myoepithelial cells were isolated and cultured to characterize their properties. The intracellular calcium concentration (Cai2+) was measured using the ratio of fura-2 fluorescence at 340 nm to that at 360 nm (F340/F360), and the contraction was simultaneously monitored by the change of fluorescence intensity at 360 nm (F360). Cultured myoepithelial cells retained their contractile machinery as in the intact tissue. NBD-phallacidin fluorescence, which marks F-actin, and electron microscopy showed abundant bundles of microfilaments in the cytoplasm. Oxytocin (> 0.1 nM) induced an increase in Cai2+ and contraction. The amplitude and time course of the Cai2+ increase were not markedly affected in the Ca2+-free solution. Nifedipine (10 microM) did not affect the response to oxytocin. ATP (>1 microM) induced an increase in Cai2+ and contraction. The response to ATP was not affected by Ca2+ removal, but was suppressed by suramin (100 microM), an antagonist of P2-purinergic receptors. The order of potency of nucleotides to increase Cai2+ was ATP = ADP > UTP > UDP. These findings indicate the presence of P2-purinergic receptors in mammary myoepithelial cells. The results suggest that stimulant-induced Cai2+ increases and contractions are due to release of Ca2+ from intracellular stores in these cells. In addition to the hormonal action of oxytocin, extracellular nucleotides may function as paracrine agents to contract myoepithelial cells in the mammary gland.