Interstitial adenosine and cellular metabolism during -adrenergic stimulation of the in situ rabbit heart

Abstract

Adenosine, derived from hydrolysis of 5'-AMP, may be involved in coupling coronary blood flow to cardiac function and metabolism. The purpose of this study was to measure interstitial fluid (ISF) adenosine and 5'-AMP levels, and cytosolic 5'-AMP in in situ rabbit heart during beta-adrenergic stimulation. Isoproterenol was infused into open chest rabbits (n = 7) at 1 and 4 micrograms.kg-1.min-1. Left ventricular ISF adenosine and 5'-AMP levels were measured using microdialysis, and energy metabolism simultaneously monitored using 31P-NMR spectroscopy. Graded beta-stimulation increased heart rate (by 50% and 70%), arterial pulse-pressure (by 55% and 45%), and the rate-pressure product (by 45% and 70%). Dialysate [adenosine] increased 300-400% from a control value of 0.44 +/- 0.13 microM. Dialysate [5'-AMP] increased 200% from a control value of 0.94 +/- 0.22 microM. Cytosolic [ATP], pH and free [Mg2+] remained stable, whereas [PCr] declined by 10-20%. Free cytosolic [5'-AMP] increased 300-400% from a control value of 0.40 microM. Competitive inhibition of ecto-5'-nucleotidase with alpha,beta-methylene-ADP (0.6 mg.kg-1.min-1) significantly reduced ISF adenosine (and enhanced ISF 5'-AMP) during beta-stimulation, but not under basal conditions. Beta-adrenergic stimulation 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 adenosine during beta-stimulation appears to occur via hydrolysis of both ISF and cytosolic 5'-AMP. ISF adenosine levels achieved during stimulation are appropriate for stimulation of cardiovascular A1 and A2 receptors.