Effect of aminophylline on adenosine and exercise dilation of rat cremaster arterioles

The present study was undertaken to evaluate the role that adenosine may play in exercise vasodilation. Responses of rat cremaster muscle arterioles to either topically applied adenosine or muscle exercise were recorded via video-microscopy before and after topical administration of aminophylline (10(-4) M). In all experiments, alpha-adrenergic receptors were blocked by pretreatment with phentolamine. Under control conditions the average concentration of adenosine required to produce a 50% dilation response (ED50) was 1.9 X 10(-6) M, and the full dose-response curve spanned 2 log units from threshold to near maximum responses. Topical aminophylline increased the ED50 for adenosine dilation to 9.1 X 10(-6) M. Stepwise increases in muscle twitch rate produced a progressive arteriolar dilation, and the dilation associated with 2-Hz exercise approached the maximal dilation observed with adenosine. Topical aminophylline, however, had no effect on the arteriolar dilation response to cremaster muscle exercise. It is unlikely that our results can be explained by any influence that exercise might have on the arteriolar sensitivity to adenosine or on the efficacy of aminophylline blockade because aminophylline had no effect on the arteriolar response to even the lowest rates of exercise tested. We conclude either that adenosine does not mediate exercise dilation in rat cremaster muscle or that interstitial adenosine levels during exercise in the presence of 10(-4) M aminophylline are significantly higher (5X) than during exercise without aminophylline.