Pharmacological classification of adenosine receptors in the sinoatrial and atrioventricular nodes of the guinea-pig

1. The effects of seven agonist and three antagonist adenosine receptor ligands were compared on the guinea-pig sinoatrial (SA) node (isolated right atrium) and atrioventricular (AV) node (perfused whole heart). Single agonist concentration-effect curves were obtained to 5'-N-ethylcarboxamidoadenosine (NECA), R(-)-N6-(2-phenylisopropyl)adenosine (R-PIA), N6-cyclohexyladenosine (CHA), 2-chloroadenosine (CADO),),S(+)-N6-(2-phenylisopropyl)adenosine (L-PIA), 2-phenylaminoadenosine (CV 1808) and N6-aminoadenosine (MeAdo). Adenosine and/or NECA curves were obtained in the absence and presence of the antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), 9-chloro-2 (2-furanyl)-5,6-dihydro-1,2,4-triazolo[1,5-c]quinazolin-5-imine (CGS15943) and N6-(endonorbornan-2-yl)-9-methyladenine (N-0861). 2. A formal comparison of the agonist and antagonist potency data was made by fitting the data to a straight line using a least squares procedure based on principal components analysis to account for the variance on both axes. The antagonist affinity estimates made on the two assays did not deviate significantly from the line of identity. 3. The agonist p[A]50 data obtained on the two assays did not deviate from the line of identity, indicating that there were no significant differences in potencies between the two assays. The p[A]50 ratio of R-PIA and S-PIA was 1.24+/-0.09 in the SA node and 1.36+/-0.11 in the AV node, indicating no difference in the stereoselectivity of the PIA isomers between the two tissues. 4. The agonist potency and antagonist affinity data obtained are consistent with previous findings showing that the AV and SA node data are pharmacologically indistinguishable and belong to the adenosine A1-receptor class. No evidence for the reported A3-receptor was found.

Pharmacological analysis of the activity of the adenosine uptake inhibitor, dipyridamole, on the sinoatrial and atrioventricular nodes of the guinea-pig

1. Dipyridamole potentiates the effects of adenosine on the heart by inhibiting adenosine uptake. The effects of dipyridamole on both adenosine and N-ethylcarboxamidoadenosine (NECA) concentration-effect (E/[A]) curves were compared on the AV node, in guinea-pig isolated perfused hearts, and on the SA node, in isolated right atria, by measuring dromotropic and chronotropic responses, respectively. In the absence of dipyridamole, adenosine was significantly more potent on the AV node than SA node (AV p[A]5, = 4.95+/-0.10. SA p[A]50=3.62+/-0.10). In contrast, NECA and adenosine in the presence of dipyridamole were approximately equiactive in the two assays (NECA: AV p[A]50=7.07+/-0.07; SA p[A]50=7.30+/-0.08: adenosine: AV p[A]50=6.49+/-0.08; SA p[A]50=6.27+/-0.05). Dipyridamole was significantly more potent in enhancing the effects of adenosine on the SA node than on the AV node (pKi values estimated by Kenakin's method (1981): AV node 8.18+/-0.14; SA node=8.75+/-0.08). 2. The difference in pKi values did not appear to be due to dipyridamole expressing other actions because concentrations of dipyridamole which saturated the adenosine transporter had no effect on the NECA E/[A] curves in either assay. However, the test of another assumption of Kenakin's method, that adenosine taken up into cells is pharmacologically inactive, failed on the AV node assay because a significant potentiating interaction was found between adenosine and NECA. The interaction was concentration-dependent, reciprocal to the extent that pre-incubation with either agonist potentiated the other and was concluded to be due to an intracellular action of adenosine as the potentiation disappeared in the presence of dipyridamole. 3. An explanatory model was developed to account for the data obtained using existing pharmacological concepts of ligand action in isolated tissue bioassays. In the model, adenosine, but not NECA, was assumed to be subject to saturable agonist uptake, an uptake which was competitively blocked by dipyridamole. Adenosine and NECA were assumed to act extracellularly at adenosine A1-receptors. In the AV node, but not the SA node, the adenosine transported into the cells was assumed to potentiate the effects of adenosine A1-receptor activation. For the AV node assay, the model predicted that potentiation of adenosine by uptake blockade is offset by a simultaneous decrease in potentiation due to the intracellular action of adenosine. All of the experimental data obtained in the study could be accounted for by the model including the apparent differences in potency of adenosine in the absence of dipyridamole and the pKi values for dipyridamole.

Long-term follow-up after attempted angioplasty of saphenous vein grafts: the Thoraxcenter experience 1981-1988

Between 1981 and 1988, 107 percutaneous transluminal coronary angioplasty (PTCA) procedures, including repeat PTCA, were performed in 84 patients with previous coronary artery bypass grafting (CABG). Fifty-nine patients underwent a first angioplasty of the vein graft alone, and 25 underwent a first PTCA of the graft and one or more native vessels. Seventeen patients underwent two procedures, four patients three procedures and one patient four procedures. In 84 first angioplasties, 133 lesions were attempted; 40 lesions in native vessels and 93 graft lesions (28 ostial stenoses, 33 shaft stenoses, and 32 stenoses at the distal anastomosis). Three patients died during their hospital stay. Two patients underwent emergency CABG. Seven patients sustained an acute myocardial infarction (AMI), among whom five underwent a PTCA of an occluded vessel. The clinical primary success rate per patient was 82%. After five years, 70% of patients were alive. At a median follow-up of 2.1 years, 41% of patients were alive and event-free (no AMI, no repeat CABG, no repeat PTCA). Symptomatic improvement was maintained in 36% of patients. Angioplasty of grafts may be an alternative to re-operation in selected patients with previous bypass surgery.