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

AbstractObjective 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 β-adrenergic stimulation. Methods Isoproterenol was infused into open chest rabbits (n = 7) at 1 and 4 μg · kg−1 · min−1. Left ventricular ISP adenosine and 5′-AMP levels were measured using microdialysis, and energy metabolism simultaneously monitored using 31P-NMR spectroscopy. Results Graded β-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 μM. Dialysate [5′-AMP]increased 200% from a control value of 0.94 ± 0.22 μM. 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 μM. Competitive inhibition of ecto-5′-nucleotidase with α,β-methylene-ADP (0.6 mg·kg−1·min−1) significantly reduced ISF adenosine (and enhanced ISP 5′-AMP) during β-stimulation, but not under basal conditions. Conclusions β-adrenergic stimulation increases ISP 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 β-stimulation appears to occur via hydrolysis of both ISF and cytosolic 5′-AMP. ISP adenosine levels achieved during stimulation are appropriate for stimulation of cardiovascular A1 and A2 receptors.