BRANCHIAL AND SYSTEMIC ROLES OF ADENOSINE RECEPTORS IN RAINBOW-TROUT - AN IN-VIVO MICROSCOPY STUDY

The purinergic branchial vasomotor control in rainbow Pout (Oncorhynch us mykiss) was studied using an epi-illumination microscope equipped w ith a water-immersion objective. Cardiac output (Q), heart rate, and d orsal (P-DA) and ventral (P-VA) aortic pressures were recorded simulta neously. Prebranchial injection of adenosine or the A(1)-receptor agon ist N-6-cyclopentyladenosine (CPA) constricted the distal portion of t he filament vasculature, which coincided with an increase of P-VA. The A(2)-receptor agonist PD-125944 was without effect. After adenosine a nd CPA injection, an overflow of blood to the secondary system was rep eatedly observed unless blood flow came to a complete stop. The lack o f a concomitant reduction of Q suggested a redistribution of blood to the secondary system and to more proximal parts of the filament. The b ranchial effects of adenosine and CPA were completely blocked by the u nspecific adenosine receptor antagonist aminophylline and the specific A(1)-receptor antagonist N-6-cyclopentyltheophylline. The results sug gest that A(1)-receptors alone mediate the branchial vasoconstriction observed. Thus the responses of the branchial vasculature to adenosine include a vasoconstriction of the filament vasculature mediated via s pecific A(1) receptors and a redistribution of blood flow to the secon dary system and to proximal parts of the filament. Additional cardiova scular effects of adenosine included decreased systemic vascular resis tance and heart rate.