Interstitial adenosine and function in rat heart in vivo: effects of adrenaline and 8-cyclopentyl-1,3-dimethylxanthine.
1. Left ventricular interstitial adenosine and cardiac function were studied in open chest rats during adrenaline stimulation and P1-purinoceptor antagonism with 8-cyclopentyl-1,3-dimethylxanthine (8-CPT). 2. Cardiac microdialysate adenosine concentration was 0.10 +/- 0.01 mumol/L (n = 24) under basal conditions, giving an estimated interstitial adenosine concentration of 0.27 mumol/L. Stimulation with 3.2 and 8.0 micrograms/kg per min adrenaline increased the rate-pressure product (heart rate x systolic blood pressure) by 72 and 157%, respectively, and increased dialysate adenosine to 0.26 +/- 0.04 and 0.65 +/- 0.11 mumol/L (n = 12), respectively (interstitial concentrations of approximately 0.70 and 1.76 mumol/L). 3. Treatment with 60 micrograms/kg per min 8-CPT did not alter basal adenosine concentrations, but potentiated elevations in dialysate adenosine during infusion of 3.2 and 8.0 micrograms/kg per min adrenaline to 0.54 +/- 0.10 and 1.30 +/- 0.22 mumol/L, respectively (n = 12). Basal function and the response to 8.0 micrograms/kg per min adrenaline were unaltered by 8-CPT, whereas elevations in heart rate and rate-pressure product during stimulation with 3.2 micrograms/kg per min adrenaline were enhanced by 8-CPT (by up to 30%). 4. Studies in isolated hearts confirmed the inhibitory potency of 8-CPT at A1 vs A2 P1-purinoceptors (e.g. pK(B) of 7.7 +/- 0.2 and 6.4 +/- 0.1 for 5'-N-ethyl carboxamidoadenosine-mediated bradycardia and vasodilatation, respectively; n = 6). Studies in intact animals verified effective A1 blockade by 60 micrograms/kg per min 8-CPT, but also revealed some inhibition of A2-mediated responses. 5. In conclusion, the data show that cardiac interstitial adenosine levels exist within a physiologically active range in vivo and increase dose-dependently during graded adrenaline stimulation. Adenosine receptor antagonism enhances elevations in interstitial adenosine and modifies functional responses to moderate, but not high, doses of adrenaline. Whether 8-CPT-dependent elevations in interstitial adenosine are due to A1 inhibition vs inhibition of A2-mediated vasodilatation requires further investigation.
Headrick
J P
JP
Department of Physiology and Pharmacology, School of Molecular Sciences, James Cook University of North Queensland, Townsville, Australia.
D000241:Adenosine / Q000032:analysis*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004837:Epinephrine / Q000494:pharmacology*; D005110:Extracellular Space / Q000737:chemistry*; D006352:Heart Ventricles / Q000737:chemistry / Q000187:drug effects; D066298:In Vitro Techniques; D008297:Male; D058915:Purinergic P1 Receptor Antagonists; D051381:Rats; D017207:Rats, Sprague-Dawley; D013806:Theophylline / Q000031:analogs & derivatives* / Q000494:pharmacology; D016277:Ventricular Function, Left / Q000187:drug effects*
D058915:Purinergic P1 Receptor Antagonists; C053907:8-cyclopentyl-1,3-dimethylxanthine; D013806:Theophylline; D000241:Adenosine; D004837:Epinephrine