Apparent reduction in baroreflex sensitivity to adenosine in conscious dogs

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Abstract
Relative effects of equihypotensive doses (-35 mmHg) of adenosine (5.0 .mu.mol/kg) and nitroglycerin (25 .mu.g/kg) on heart rate and, therefore, baroreflex sensitivity were studied in conscious dogs. Nitroglycerin increased heart rate 133 .+-. 24% from 78 .+-. 5.5 beats/min, whereas adenosine increased heart rate only 79 .+-. 16% from 78 .+-. 5.2 beats/min (P < 0.01). Injection of nitroglycerin during combined .beta.-adrenergic and muscarinic receptor blockades caused arterial pressure to fall 38 .+-. 3.4% from 107 .+-. 3.2 mmHg without any significant change in heart rate (3.8 .+-. 3.8 from 162 .+-. 9.2 beats/min). During combined .beta.-adrenergic and muscarinic receptor blockades adenosine also reduced arterial pressure 45 .+-. 2.7% from 106 .+-. 2.9 mmHg but unexpectedly reduced heart rate as well as by 37 .+-. 1.7% from 160 .+-. 9.7 beats/min. This bradycardia reflected an effect on the sinotrial (SA) node rather than an induction of heart block, since the R-R interval increased by 70 .+-. 7.8% from 371 .+-. 20 ms (P < 0.01), while the P-R interval increased only 13 .+-. 2.3% from 97 .+-. 7.2 ms (P < 0.05) with no electrocardiographic evidence of nonconducted beats. Arterial plasma adenosine levels were 43 .+-. 5 nmol/ml at this time. Adenosine also caused bradycardia during muscarinic blockade alone (-43 .+-. 3.4% for 201 .+-. 6.4 beats/min) and following bilateral vagal section (-33 .+-. 1.9% from 151 .+-. 5.9 beats/min). In summary, adenosine appears to alter normal baroreflex function in the conscious dog by reducing the tachycardia that normally follows a fall in systemic arterial pressure. This action of adenosine, however, is directly on the SA node and not on the neural mechanisms that make up the baroreflexes.