The role of calcium in the regulation of cyclic nucleotide levels in brain slices of rat and guinea pig

Abstract

1. In rat cerebral cortical slices, the addition of EGTA to acutely lower extracellular calcium concentrations greatly reduced the accumulation of cyclic AMP elicited by norepinephrine while having little effect on the response to adenosine or N6-phenylisopropyladenosine. The inhibitory effect of EGTA on the response to norepinephrine was reversible. A prolonged 30 min preincubation of rat cortical slices with EGTA or merely with calcium-free medium produced a marked increase in basal levels of cyclic AMP which was inhibited by theophylline or adenosine deaminase. The increase in basal levels of cyclic AMP thus appears due to increased release of adenosine. The released adenosine potentiated the response to exogenous biogenic amines and thus complicated studies on the direct effect of extracellular calcium in cyclic AMP-systems in brain sclices. Elimination of released adenosine with adenosine deaminase reduced responses to norepinephrine in control slices and blocked responses to norepinephrine in EGTA-treated slices. Adenosine deaminase also reduced responses to phenylisopropyladenosine. 2. In guinea pig cortical slices the addition of EGTA to acutely reduce extracellular calcium increased basal levels of cyclic AMP and the response to norepinephrine, adenosine and phenylisopropyl-adenosine. The response to histamine was unchanged. Elimination of released adenosine with adenosine deaminase prevented the EGTA-elicited increase in basal levels of cyclic AMP and completely blocked the response to norepinephrine or histamine. Adenosine deaminase reduced the response to the amines in control slices but not that due to phenylisopropyl-adenosine. Cyclic GMP levels were increased 2 to 3-fold in guinea pig cerebral cortical slices by norepinephrine, histamine, and adenosine. These responses were blocked by the addition of EGTA. 3. The effects of the potent phosphodiesterase inhibitor, isobutylmethylxanthine, on cyclic AMP levels in brain slices both in the presence and absence of extracellular calcium appear, like the effects of theophylline, to primarily reflect inhibition of adenosine-responses rather than inhibition of phosphodiesterase. The effects of another phosphodiesterase inhibitor, Ro 20-1724, on cyclic AMP levels appear to reflect both a Ro 20-1724-elicited increase in net efflux of adenosine and inhibition of phosphodiesterase. 4. The results indicate that a) the presence of extracellular calcium is critical for norepinephrine and histamine-elicited accumulations of cyclic AMP in brain slices, but not for responses to adenosine, and that b) released adenosine can, at least partially, substitute for extracellular calcium in maintaining responsiveness of amine-sensitive cyclic AMP-generating systems in brain slices.