title: Ca-antagonistic effects of adenosine in guinea pig atrial cells.
abstract: In atrial myocytes of the guinea pig, the effects of adenosine (Ado) and acetylcholine (ACh) on Ca currents (ICa) were investigated with the patch-electrode whole cell clamp technique. ICa was dissected from net currents by blocking K currents (IK) with intra- and extracellular Cs ions. ICa was considered as "basal" ICa, since it was not prestimulated by beta-agonists (isoproterenol). T-channel Ca currents were insensitive to Ado or ACh. The antagonism of L-channel Ca currents was maximal with 10 microM Ado or 3 microM ACh, which reduced basal ICa by 35%. From the concentration dependence, a dissociation constant (KD) value of 1.1 microM Ado and a Hill coefficient of -3 were obtained. Ado and ACh were not additive but saturative in reducing basal ICa. Reduction of basal ICa did not modify inactivation time-course, steady-state activation or inactivation, suggesting that Ado reduces the number of functional Ca channels. In myocytes with unblocked IK (KCl electrodes), 3 microM Ado (1 microM ACh) reduced ICa by 30% but increased IK by 300%. It is concluded that the K-agonistic rather than the Ca-antagonistic effect accounts for hyperpolarization as well as for most of the shortening of the action potential and the negative inotropy.
authors.affiliation: Department of Applied Physiology, University of Cologne, Federal Republic of Germany.
authors.forename: E
authors.initials: E
authors.lastname: Cerbai
authors.forename: U
authors.initials: U
authors.lastname: Klöckner
authors.forename: G
authors.initials: G
authors.lastname: Isenberg
mesh_terms: D000241:Adenosine / Q000494:pharmacology*; D000818:Animals; D016275:Atrial Function; D002118:Calcium / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D002478:Cells, Cultured; D002586:Cesium / Q000494:pharmacology; D006168:Guinea Pigs; D006321:Heart / Q000502:physiology*; D006325:Heart Atria / Q000187:drug effects; D007545:Isoproterenol / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology
chemical_list: D015220:Calcium Channels; D015221:Potassium Channels; D002586:Cesium; D000241:Adenosine; D007545:Isoproterenol; D002118:Calcium