ADRENOCEPTOR HETEROGENEITY IN HUMAN WHITE ADIPOCYTES DIFFERENTIATED IN CULTURE AS ASSESSED BY CYTOSOLIC-FREE CALCIUM MEASUREMENTS

Changes in intracellular calcium concentration [Ca2+](i) in response t o norepinephrine (NE) and to various adrenergic agonists were monitore d by dual excitation microfluorimetry in single human adipocytes diffe rentiated in culture and loaded with fura-2 acetoxymethyl ester (fura- 2 AM). The addition of NE elicited increases in [Ca2+](i) that were, d epending on the cell, (1) either rapid (time to peak: 9 +/- 3 s), larg e, and transient; or (2) slow (time to peak: 125 +/- 8 s), small, and sustained. The rapid and large [Ca2+](i) response, which was inhibited by 90% by the alpha(1)-antagonist prazosin and only by 20% by the non -specific beta antagonist(-)-propranolol, was considered to be mediate d by the alpha(1)-adrenoceptor. In face, an alpha(1A)-adrenoceptor was found to be expressed in human white adipose tissue. Consecutive addi tions of beta-agonists specific for each subtype of alpha-adrenoceptor enabled the characterization of four cell populations with different response patterns: 47% of the cells had alpha(1)- and beta(1)-, beta(2 )- and beta(3)-induced [Ca2+](i) responses; 29% had only beta(1)-, bet a(2)-, beta(3)-responses; 14% had alpha(1)- and beta(3)-responses, and 10% had only an alpha(1)-response. Taken together, these results show that in differentiated human adipocytes: (1) alpha(1)- and beta-adren ergic stimulations induce [Ca2+](i) increases with different kinetics and amplitudes; (2) there is a beta(3)-adrenergic response similar to the beta(1)- or beta(2)-adrenergic responses; and (3) there is a marke d adrenoceptor heterogeneity.