Effects of islet-activating protein on insulin- and isoprenaline-stimulated glucose transport in isolated rat adipocytes

The effects of islet-activating protein (IAP), a Bordetella pertussis toxin, on insulin- and isoprenaline-stimulated glucose transport were studied in isolated rat adipocytes. Basal as well as insulin-stimulated glucose transport were not affected when cells were pretreated with IAP. In contrast, IAP pretreatment abolished the stimulatory effect of isoprenaline. When IAP-pretreated cells were exposed to a combination of insulin and isoprenaline, the catecholamine significantly reduced the stimulatory effect of insulin. Since IAP is supposed to specifically block the inhibitory component Ni of adenylate cyclase, the results suggest that: (a) the effect of insulin is unrelated to the regulation of adenylate cyclase; (b) isoprenaline may exert both stimulatory and inhibitory effects depending on activation of Ni. The inhibitory regulation of adenylate cyclase may thus be a pivotal link in the regulation of glucose transport.