title: Na+-Ca2+ exchange current from rabbit isolated atrioventricular nodal and ventricular myocytes compared using action potential and ramp waveforms.
abstract: We measured and compared Na-Ca exchanger current (INa-Ca) from rabbit isolated ventricular and atrioventricular (AV) nodal myocytes, using action potential (AP) and ramp voltage commands. Whole cell patch-clamp recordings were made at 35-37 degrees C; INa-Ca was measured as 5 mM nickel (Ni)- sensitive current with major interfering voltage and calcium-activated currents blocked. In ventricular cells a 2-s descending ramp elicited INa-Ca showing outward rectification and a reversal potential (Erev) of -13.1 +/- 1. 2 mV (n = 12; mean +/- SEM). With a ventricular AP as the voltage command, the profile of INa-Ca followed the applied waveform closely. The current-voltage relation during AP repolarization was almost linear and showed an Erev of -38.3 +/- 5.3 mV (n = 6). As INa-Ca depended on the applied voltage waveform, comparisons between the two cell types utilized the same command waveform (a series of AV nodal APs). In ventricular myocytes this elicited INa-Ca that reversed near -38 mV and was inwardly directed during the pacemaker potential. This command was also applied to AV node cells; mean INa-Ca density at all voltages encompassed by the AP (-70 to +30 mV) did not differ significantly from that in ventricular myocytes (P > 0.05, ANOVA). This finding was confirmed using brief (250 ms) voltage ramp protocols (P > 0.1 ANOVA). These data represent the first direct measurements of AV nodal INa-Ca and suggest that the exchanger may be functionally expressed to similar levels in the two cell types. They may also suggest a possible role for INa-Ca during the pacemaker potential in AV node as inward INa-Ca was observed over the pacemaker potential range even with bulk internal Ca buffered to a low level.
authors.affiliation: Department of Physiology & Cardiovascular Research Laboratories, School of Medical Sciences, University Walk, Bristol, UK.
authors.forename: M K
authors.initials: MK
authors.lastname: Convery
authors.forename: J C
authors.initials: JC
authors.lastname: Hancox
mesh_terms: D000200:Action Potentials; D000818:Animals; D001283:Atrioventricular Node / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D004594:Electrophysiology; D006352:Heart Ventricles / Q000378:metabolism; D066298:In Vitro Techniques; D017136:Ion Transport; D008297:Male; D009206:Myocardium / Q000378:metabolism*; D011817:Rabbits; D012964:Sodium / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger / Q000378:metabolism
chemical_list: D019831:Sodium-Calcium Exchanger; D012964:Sodium; D002118:Calcium