Prostaglandin‐sensitive adenylyl cyclase of cultured preadipocytes and mature adipocytes of the rat: Probable role of G_i in determination of stimulatory or inhibitory action

Preadipocytes of rats were obtained from the stromal‐vascular fraction of collagenase‐digested perirenal fat pads and grown in serum‐containing medium. By day 8 of culture the cells reached confluence and by 12 days were lipid‐laden. The adenylyl cyclase of the plasma membranes was compared to that of mature fat cells. Unlike the membranes from adipocytes, the preadipocytes showed adenylyl cyclase activity that was stimulated by GTP. Stimulation of preadipocyte membranes by Gpp(NH)p, NaF, and forskolin was comparable to that of membranes from adipocytes, but the response to epinephrine and isoproterenol was minimal (approximately 1.5‐fold for preadipocytes vs. 4–5‐fold for adipocytes). In contrast, GTP‐dependent stimulation of adenylyl cyclase of preadipocytes by PGE₁ was nearly 8‐fold. Stimulation occurred even in the presence of both GTP and 140 mM NaCl, a condition that leads to inhibition by PGE₁ of adenylyl cyclase in membranes of adipocytes. Other characteristics of the adenylyl cyclase of preadipocyte membranes that differ from those of adipocytes include lack of inhibition by GTP of forskolin‐activated activity, and, following treatment with pertussis toxin, enhanced stimulation by PGE₁. ADP‐ribosylation of G_i and G_s with pertussis and cholera toxins, respectively, indicated that the membranes of preadipocytes contained only 5–11% of the G_i of adipocytes and a much lower ratio of G_i:G_s. These findings suggest that cultured preadipocytes have an incompletely developed G_i pathway that may account for the stimulatory effect of prostaglandins on the adenylyl cyclase of these cells as opposed to the inhibitory action of PG in mature fat cells.