The effect of magnesium and calcium on the physiological properties of certain purine derivatives

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Electrophysiologic effects of adenosine triphosphate and adenosine on the mammalian heart: clinical and experimental aspects

Adenosine triphosphate (ATP) and adenosine have strong negative chronotropic and dromotropic effects on the mammalian heart. The sensitivity of the sinus node and the atrioventricular node to ATP and adenosine manifests pronounced variability among species. For more than three decades, ATP has been used routinely in Europe in the acute therapy of paroxysmal supraventricular tachycardia. Preliminary clinical trials with adenosine in the United States suggest that this compound may have a similar therapeutic value. The exact mechanisms of action of ATP and adenosine on the mammalian heart are still not fully known. However, the vast clinical experience indicates that ATP, and probably also adenosine, can be safely and repetitively used in the acute therapy of paroxysmal supraventricular tachycardia.

Inotropic responses of the frog ventricle to adenosine triphosphate and related changes in endogenous cyclic nucleotides

1. A study has been made of a well documented but poorly understood response of the isolated frog ventricle to treatment with exogenous adenosine 5' triphosphate (ATP). Measurements of membrane potential, isometric twitch tension and levels of endogenous 3',5'-cyclic nucleotides have been made at various times during the ATP-induced response. 2. ATP elicits a characteristic triphasic response, which comprises an initial, abrupt increase in contractility, rising to a maximum within a few beats (first phase); followed by a period when the twitch amplitude falls, sometimes to below the control level (second phase); and superceded by a more slowly developing and longer-lasting increase in contractile force (third phase). The response is unaffected by atropine, propranolol or phentolamine. However, the prostaglandin synthetase inhibitor indomethacin depresses the first phase and entirely suppresses the third phase. 3. The inotropic effects of ATP are accompanied by changes in the shape of the action potential. These effects are dose-related. The duration of the action potential (D-30mV) and its positive overshoot (O) are increased during all phases of the response, for [ATP]o's up to 10(-5) M. However, at higher [ATP]o's, D-30mV and O ar both reduced during the second phase (but not the first or third phase), when isometric twitch tension is also depressed. The relationship between action potential duration and twitch tension (P) for different [ATP]o's is linear for all three phases of the response, but the slopes of the curves (delta P/delta D) are markedly different, indicating that the sensitivity of the contractile system to membrane depolarization is not constant, but varies continuously throughout the response. 4. ATP has a potent stimulatory effect on the metabolism of endogenous 3',5'-cyclic nucleotides. The time courses of the changes in adenosine 3','5-cyclic monophosphate (3',5'-cyclic AMP) and guanosine 3',5'-cyclic monophosphate (3',5'-cyclic GMP) are complex, but the accompanying change in isometric twitch tension is paralleled closely by corresponding changes in the ratio 3',5'cyclic AMP:3',5'-cyclic GMP. 5. It is concluded that ATP exerts a dual effect on the ventricle and that the contractile response is regulated by changes in the metabolism of 3',5'-cyclic nucleotides. The effects of indomethacin indicate a possible involvement of prostaglandins in mediating the ATP response. It is suggested that the initial effect of ATP on the ventricle is to increase the permeability of the fibres to Ca2+. 6. The relationship between 3',5' cyclic nucleotide levels and ventricular contractility is discussed. It is postulated that the antagonistic effects of 3',5'-cyclic AMP and 3',5'-cyclic GMP are expressed at the level of certain phosphoproteins which regulate both the availability of Ca2+ and the sensitivity of the contractile proteins to Ca2+.

An estimate of adenosine triphosphate release into the venous effluent from exercising human forearm muscle

1. Human subjects performed a sustained contraction of the forearm muscles for 4 min in the presence of arterial and venous occlusion.2. The contraction was maintained at 5% of the maximum voluntary contraction, a tension during which the muscle blood flow might be expected to increase by about three times (Lind & McNicol, 1967).3. Adenosine triphosphate (ATP) was identified in the venous effluent from occluded exercising forearm, but not in the venous effluent from occluded forearm without exercise.4. The rate of degradation of ATP was assessed in plasma at 37 degrees C, with an estimate of the percentage loss occurring between sampling and testing. This enabled the rate of appearance of ATP in the blood at the time of exercise to be calculated as approximately 7.5-10.5 mug/min (14-20 n-mole/min). These amounts are compared with 16 mug/min that was infused intra-arterially into human forearm to cause a threefold increase in blood flow (Duff, Patterson & Shepherd, 1954).5. It is likely that the ATP detected in the venous effluent has active muscle as the source; if so, then the amounts calculated to be released could satisfy the vasodilator requirements of active skeletal muscle.6. The effects of circulating ATP on respiration and coronary blood flow during exercise is discussed, including the role it may play locally in the production of ischaemic pain.