Modulation of insulin release by adenosine A1 receptor agonists and antagonists in INS-1 cells: The possible contribution of 86Rb+ efflux and 45Ca2+ uptake

Due to the lack of specific agonists and antagonists the role of adenosine receptor subtypes with respect to their effect oil the insulin secretory system is not well investigated. The A, receptor may be linked to different 2nd messenger systems, i.e. cAMP, K+- and Ca-45(2+) channel activity. Partial A(1) receptor agonists are going to be developed in order to improve diabetes (increase in insulin sensitivity, lowering of FFA and triglycerides). In this study newly synthesized selective A] receptor agonists and antagonists were investigated thereby integrating three parameters, insulin release (RIA), Ca-45(2+) uptake and Rb-86(+) efflux (surrogate for K+ efflux) of INS-1 cells, an insulin secretory cell line. The presence of A(1)-receptors was demonstrated by Western blotting. The receptor nonselective adenosine analogue NECA (5'-N-ethylcarboxyamidoadenosine) at high concentration (10 mu M) had no effect on insulin release and Ca-45(2+) uptake which could be interpreted as the sum of effects mediated by mutual antagonistic adenosine receptor subtypes. However, an inhibitory effect mediated by At receptor agonism was detected at 10 nM NECA and Could be confirmed by adding the A I receptor antagonist PSB-36 (1-butyl-8-(3-noradamantyl)-3-(3-hydroxy-propyl)xanthine). NECA inhibited Rb-86(+) efflux which, however, did not fit with the Simultaneous inhibition of insulin secretion. The selective A, receptor agonist CHA (N-6-cyclohexyladenosine) inhibited insulin release; the simultaneously increased Ca2+ uptake (nifedipine dependent) and inhibition of Rb-86(+) efflux did not fit the insulin release data. The CHA effect (even the maximum effect at 50 mu M) can be increased by 10 mu M NECA indicating that CHA and NECA have nonspecific and physiologically non-relevant effects on Rb-86(+) efflux in addition to their A(1)-receptor interaction. Since PSB-36 did not influence the NECA-induced inhibition of Rb-86(+) efflux, the NECA effect is not mediated by potassium channel-linked At receptors. The nonselective adenosine receptor antagonist caffeine increased insulin release which was reversed by CHA its expected when hypothesizing that both act via A, receptors in this case. In conclusion, stimulation of A, receptors by receptor selective and nonselective compounds reduced insulin release which is not coupled to opening of potassium channels (Rb-86(+) efflux experiments) or inhibition of calcium channels (Ca-45(2+) uptake experiments). It may be expected that of all pleiotropic 2nd messengers, the cAMP system (not tested here) is predominant for A I receptor effects and the channel systems (K+ and Ca2+) are of minor importance and do not contribute to insulin release though being coupled to the receptor in other tissues. Copyright (C) 2008 John Wiley & Sons, Ltd.