The adenosine hypothesis revisited: relationship between purine release and coronary flow in isolated rat heart.

We evaluated the adenosine hypothesis through a new approach, based on the study of the relationship between coronary flow or resistance and purine release, which is an accurate index of myocardial adenosine release. Isolated rat hearts were perfused at different work loads, in hypoxic conditions and after a short period of global ischaemia. When the results of all experiments were considered together, purine release was significantly but weakly related to coronary flow and coronary resistance (r = 0.416 v coronary flow, r = 0.378 v the reciprocal of coronary resistance, p less than 0.01). Closer relationships were obtained within the three subgroups: the correlation coefficients increased to 0.819 and 0.835 (p less than 0.001) in the hearts perfused at different work loads with normal oxygen supply, to 0.701 and 0.757 (p less than 0.02 and p less than 0.01) in the hypoxic hearts, and to 0.897 and 0.978 (p less than 0.02 and p less than 0.01) in the hearts recovering from ischaemia. The relationships between purine release and coronary flow or resistance were significantly different in the three subgroups (p less than 0.001): at any value of purine release coronary resistance was highest during hypoxia and lowest after ischaemia, while the opposite was true for coronary flow. We suggest that the adenosine hypothesis is converted into a "weaker" statement: adenosine is involved in the adjustment between heart performance and coronary resistance but other factors contribute to the regulation of coronary flow, and/or affect the response to adenosine.

Cancer incidence and mortality in patients with heart disease. Effect of oral anticoagulant therapy.

Cancer incidence and mortality were reviewed in patients (683) who, during the period 1969-1988, had been attending the Cardiological Center of Pisa University for more than 1 year for valvular (494), ischemic (183), or myocardial (6) disease. Oral anticoagulant therapy (tromexan, acenocoumarol or warfarin) was administered to 312 of these 693 patients and regulated to prolong prothrombin time to a value between 20% and 40% of normal controls. The duration of treatment ranged from 1 to 14 years, with a mean of 4 years. As clinical and radiological controls were performed on all the patients at regular intervals (2-12 months), cancer incidence and mortality were recorded. Cancer incidence and mortality in the 312 patients treated with anticoagulants were compared with that of the 381 patients who did not receive this therapy. Furthermore, cancer mortality in the patients on anticoagulants was compared to that expected on the basis of national tumor registry rates. The age of the patients varied from 20 or under to 80 or over in both groups. The total observation period was 1415 patient-years (555 for males and 860 for females) in the former and 1617 patient-years (735 males and 882 females) in the latter. The proportion of the patient-years of the men over 45 (with the highest risk of cancer mortality) was higher in the group treated with anticoagulants (83%) than in the controls (72%) (p < .001). The proportion of the patient-years of the women over 45 was also higher in the former (84% vs 62%; p < .001). Six cancers were observed in the patients treated with oral anticoagulants (3 men, 3 women), while 12 cancers occurred in the control group (9 men, 3 women). There were 3 deaths in the former (1 man, 2 women) and 6 in the latter (5 men, 1 woman). On the basis of the national tumor registry rates, deaths expected in men and women on oral anticoagulants were 3 and 2. These data are compatible with the hypothesis that oral anticoagulants might reduce cancer incidence and mortality in humans.

Effect of amrinone in the working rat heart: influence of ischaemic damage, adenosine and calcium.

The action of amrinone on the isolated working rat heart was studied. In basal conditions up to 400 mg/l amrinone behaved as a pure chronotropic agent, raising the heart rate by 23%. In ischaemia-damaged hearts 100 mg/l amrinone had a true inotropic action, provoking significant increases in cardiac output (11%), aortic flow (10%) and double product (9%). The addition of 10 microM adenosine, which inhibits the inotropic action of catecholamines, did not modify the response to amrinone. However, when calcium was reduced from 2.5 to 1.25 mM, the inotropy of ischaemia-damaged hearts was not significantly affected by amrinone, and its chronotropic action was reduced. It is concluded that after acute ischaemic damage amrinone increases heart performance even in a poorly responsive species in which no inotropic effect can be produced under basal conditions. In this model its action is resistant to adenosine but depends on the availability of a sufficient amount of extracellular calcium.

Adenosine binding sites in pig ventricular sarcolemma and interaction with calcium channels.

Two classes of adenosine binding sites were identified in pig ventricular sarcolemma using 5'-N-[3H]ethylcarboxamideadenosine ([3H]NECA) as radioligand (Bmax = 88 fmol/mg protein, Kd = 28 nM; Bmax = 7100 fmol/mg protein, Kd = 730 nM). Competition experiments indicated that the higher affinity group had the pharmacological characteristics of the A2 adenosine receptors. The lower affinity binding sites may correspond to the A3 adenosine receptors demonstrated in rat brain. Results with the calcium antagonist [3H]nitrendipine suggest that adenosine exerts some of its effects on the heart also through a direct binding to specific calcium-channel subtypes.

Purine release from isolated rat heart: a new approach to the study of energy metabolism.

The rate of release of purines (adenosine, inosine, hypoxanthine, xanthine and uric acid) from isolated working rat hearts was measured and compared to tissue concentrations of high energy phosphate compounds. Hearts were subjected to different workloads, and perfusions were performed: with normal oxygen supply (group 1); with the addition of insulin to the standard perfusion buffer, which contained glucose as energy source (group 2); in hypoxic conditions (group 3). In each group purine release increased (P less than 0.01) at higher workload and was closely related to indices of mechanical performance such as cardiac output or minute work (r = 0.902 and 0.858 in group 1, r = 0.902 and 0.851 in group 2, r = 0.851 and 0.881 in group 3, P less than 0.001 in each case). Work had no effect on adenine nucleotides but produced a significant (P less than 0.01) reduction in phosphocreatine/creatine ratio. The comparison of different groups showed that at any level of heart performance purine release was higher (P less than 0.001) in group 3 vs. group 1, and lower (P less than 0.001) in group 2 vs. group 1. High energy phosphates were reduced in group 3 vs. group 1 but were unchanged in group 2 vs. group 1. We conclude that in the isolated heart purine release is directly related to the rate of energy consumption, and inversely related to the rate of energy production. Purine release provides a sensitive method to evaluate myocardial energy metabolism, which is more sensitive than measurement of high energy phosphates.

Protection of isolated rat heart from oxidative stress by exogenous creatine phosphate.

The influence of exogenous creatine phosphate (CP) on peroxidative heart injury was investigated in two experimental models: isolated working rat hearts and myocardial membrane preparations. In the first model the addition of 190 microM hydrogen peroxide to the perfusion buffer caused a marked decrease of aortic flow, minute work and peak aortic pressure, and leakage of intracellular enzymes. In the presence of 10 mM CP the hemodynamic damage produced by the same concentration of hydrogen peroxide was significantly lower and enzyme release was also remarkably reduced. The protection was concentration-dependent and the whole structure of the molecule was required since creatine was found to be ineffective. In the absence of hydrogen peroxide, CP and creatine did not affect heart performance. In microsomal membrane preparations CP decreased the formation of thiobarbituric acid-reactive material (malonaldehyde) induced by hydrogen peroxide in the presence of ferrous ions. This protection was concentration-dependent and occurred at physiological concentrations of CP. Also in this experimental model creatine had no effect and creatine plus inorganic phosphate was much less active than CP. The influence of CP on oxidative heart stress could account for the beneficial effect of this substance in different models of ischemic injury.