INTERSTITIAL ADENOSINE AND CELLULAR-METABOLISM DURING BETA-ADRENERGICSTIMULATION OF THE IN-SITU RABBIT HEART

Objective: Adenosine, derived from hydrolysis of 5'-AMP, may be involv ed in coupling coronary blood flow to cardiac function and metabolism, The purpose of this study was to measure interstitial fluid (ISF) ade nosine and 5'-AMP levels, and cytosolic 5'-AMP in in situ rabbit heart during beta-adrenergic stimulation. Methods: Isoproterenol was infuse d into open chest rabbits (n = 7) at 1 and 4 mu g . kg(-1). min(-1). L eft ventricular ISF adenosine and 5'-AMP levels were measured using mi crodialysis, and energy metabolism simultaneously monitored using P-31 -NMR spectroscopy. Results: Graded beta-stimulation increased heart ra te (by 50% and 70%), arterial pulse-pressure (by 55% and 45%), and the rate-pressure product (by 45% and 70%). Dialysate [adenosine] increas ed 300-400% from a control value of 0.44 +/- 0.13 mu M. Dialysate [5'- AMP] increased 200% from a control value of 0.94 +/- 0.22 mu M. Cytoso lic [ATP], pH and free [Mg2+] remained stable, whereas [PCr] declined by 10-20%. Free cytosolic [5'-AMP] increased 300-400% from a control v alue of 0.40 mu M. Competitive inhibition of ecto-5'-nucleotidase with alpha,beta-methylene-ADP (0.6 mg . kg(-1). min(-1)) significantly red uced ISF adenosine (and enhanced ISF 5'-AMP) during beta-stimulation, but not under basal conditions. Conclusions: beta-adrenergic stimulati on increases ISF adenosine levels and depresses bioenergetic state in in situ rabbit heart without altering [ATP], pH or [Mg2+], indicating an absence of ''demand'' ischemia. ISF adenosine originates primarily from cytosolic 5'-AMP under basal conditions whereas increased adenosi ne during beta-stimulation appears to occur via hydrolysis of both ISF and cytosolic 5'-AMP. ISF adenosine levels achieved during stimulatio n are appropriate for stimulation of cardiovascular A(1) and A(2) rece ptors.