Forskolin refractoriness. Exposure to the diterpene alters guanine nucleotide-dependent adenylate cyclase and calcium-uptake activity of cells cultured from the rat aorta

Publisher Website
Google Scholar
Abstract
Cells with the morphological properties of endothelial cells were cultured from the rat aorta. The cultured cells accumulated 45Ca2+ from the medium in a manner which was stimulated by forskolin and by 8-bromo-cyclic AMP. Pretreating the cultures for 20 h with forskolin diminished forskolin-dependent Ca2+-uptake activity. Adenylate cyclase activity of cultured cell homogenates was stimulated by guanosine 5''-[.beta.,.gamma.-imido]triphosphate (p[NH]ppG) and forskolin, and by isoprenaline in the presence, but not in the absence, of guanine nucleotide. p[NH]ppG increased forskolin sensitivity and caused a leftward shift in the forskolin dose-response curve. Pretreating the cultured cells with forskolin for 20 h, conditions that decreased forskolin-dependent Ca2+ uptake, increased basal and guanine nucleotide-dependent adenylate cyclase activity, but not forskolin-dependent activity determined in the absence of p[NH]ppG. Forskolin pretreatment diminished p[NH]ppG''s capacity to increase forskolin sensitivity, but did not have a significant effect on either the sensitivity of adenylate cyclase to p[NH]ppG or its responsiveness to isoprenaline. These results suggest that the Ca2+-uptake mechanism is cyclic AMP-dependent and that guanine nucleotides mediated forskolin-dependent cyclic AMP production by the intact cells. In addition, there may be different guanine nucleotide requirements for hormone-receptor coupling and forskolin activation.