Characterization of oscillations in cytosolic free Ca2+ concentration and measurement of cytosolic Na+ concentration changes evoked by angiotensin II and vasopressin in individual rat aortic smooth muscle cells. Use of microfluorometry and digital imaging

Dual wavelength microfluorometry was used to characterize the changes in cytosolic free Ca2+ concentration ( Ca2+]i) in individual cultured rat aortic vascular smooth muscle cells (VSMC). Angiotensin II (ANG II) at 10(-8) M induced a transient rise in Ca2+]i from 43 +/- 2 to 245 +/- 23 nM, lasting for approximately 60 s (n = 42). In half of the population, discrete oscillations in Ca2+]i of smaller amplitude occurred after the initial Ca2+]i peak, with a period of 58 +/- 8 s and a maximum height of 132 +/- 24 nM. A similar oscillatory pattern was observed with arginine vasopressin (AVP). The oscillations depended upon the presence of extracellular Ca2+. Cytosolic free Na+ concentration (Na+]i) in VSMC was also measured using the fluorescent Na+ probe sodium-binding benzofuran isophthalate. ANG II induced a gradual and sustained elevation of Na+]i, from 24.0 +/- 6.2 to 36 +/- 9.7 mM. In response to AVP, Na+]i rose to 41.0 +/- 11.6 mM. Video imaging of individual VSMC, with on-line ratio calibration of Ca2+]i, revealed an inhomogeneous distribution of Ca2+ within the cell. Ca2+] in the nucleus was invariably lower than in the cytoplasm in resting cells. In the cytoplasm, there were small regions in which Ca2+] was elevated, or "hot spots." In Ca(2+)-containing medium, the initial rise in Ca2+]i triggered by ANG II and AVP appeared to emanate from the hot spots and to spread evenly throughout the cytoplasm. Between Ca2+]i oscillations, Ca2+ retreated back to the original hot spots. This study demonstrates the cellular and subcellular heterogeneity of Ca2+]i both in resting VSMC and during stimulation by ANG II and AVP and reports the direct measurement of Na+]i in VSMC. The results suggest an action of Ca2+ in both the initial and sustained phases of the response in VSMC and a link between changes in Ca2+]i and Na+]i.