Mechanism of extracellular ATP-induced increase of cytosolic Ca2+ concentration in isolated rat ventricular myocytes.
1. Changes in the cytosolic Ca2+ concentration ([Ca2+]i) of isolated rat ventricular myocytes in suspension were measured in response to extracellular ATP using the fluorescent Ca2+ indicators Quin-2 and Fura-2. 2. ATP produced a concentration-, time- and Mg(2+)-dependent, biphasic increase of [Ca2+]i whereas slowly hydrolysable ATP analogues produced a slow, monophasic increase of [Ca2+]i and the non-hydrolysable ATP analogues were without effect. 3. Extracellular Ca2+ was required for the ATP-induced increase of [Ca2+]i and pre-treatment of the cells with caffeine, ryanodine, verapamil or nimodipine partially inhibited the [Ca2+]i increase. 4. Whole-cell patch-clamp experiments revealed that ATP activated an ionic current that had a linear current-voltage relationship with a reversal potential near O mV. Quinidine, a putative P2 purinergic receptor blocker, abolished the ATP-activated current. The ATP-activated current was Mg2+ dependent. 5. Associated with the ATP-activated current was cellular depolarization. In a physiological solution, ATP depolarized cells to the threshold for the firing of action potentials. In the presence of the voltage-activated ion channel blockers tetrodotoxin, 4-aminopyridine, caesium and nitrendipine, ATP depolarized cells to -44 +/- 6 mV from a resting potential of -66 +/- 4 mV (n = 11). 6. Sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis and autoradiography demonstrated that extracellular ATP stimulated the phosphorylation of several extracellular membrane-bound proteins. The phosphorylation of these proteins was concentration, time and Mg2+ dependent. Pre-treatment of cells with the slowly hydrolysable ATP analogues inhibited the ATP-induced phosphorylation. Adenosine 5'-O-3-thiotriphosphate (ATP gamma S) thiophosphorylated proteins with the same apparent molecular weight as the proteins phosphorylated by ATP. 7. These results suggest that the ATP-induced increase of [Ca2+]i is a result of the activation, possibly by protein phosphorylation, of a novel ion channel carrying inward current. The ATP-activated channel may be permeable to Na+ and Ca2+ and causes [Ca2+]i to rise. More importantly, this inward current depolarizes the cell to the threshold of inducing spontaneous firing of action potentials. The firing of action potentials results in the influx of Ca2+ through L-type Ca2+ channels which would trigger Ca2+ release from the sarcoplasmic reticulum and lead to the increase in [Ca2+]i.
Christie
A
A
Department of Pharmacology, University of Rochester, School of Medicine and Dentistry, NY 14642.
Sharma
V K
VK
Sheu
S S
SS
D000200:Action Potentials / Q000187:drug effects; D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D002478:Cells, Cultured; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism*; D005110:Extracellular Space / Q000378:metabolism*; D006321:Heart / Q000187:drug effects; D006352:Heart Ventricles; D008274:Magnesium / Q000378:metabolism; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D009553:Nimodipine / Q000494:pharmacology; D051381:Rats; D012433:Ryanodine / Q000494:pharmacology; D014700:Verapamil / Q000494:pharmacology
D015220:Calcium Channels; D012433:Ryanodine; D002110:Caffeine; D009553:Nimodipine; D000255:Adenosine Triphosphate; D014700:Verapamil; D008274:Magnesium; D002118:Calcium