Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, C75
Interrelationship between conduction velocity, intracellular Ca2+ and gap junction resistance in ventricular myocardium.
R. I. Jabr1, S. Salvage1, P. S. Dillon2, C. H. Fry1
1. Division of Clinical Medicine, University of Surrey, Guildford, United Kingdom. 2. Institute of Urology, University College, London, United Kingdom.
Arrhythmogenesis could originate from local alteration in action potential (AP) conduction caused by changes to gap junction resistance (Rj). Cardiac arrhythmias are associated with a rise of intracellular Ca2+ concentration ([Ca2+]i) and an increased activity of Ca2+-calmodulin dependent protein phosphatase, calcineurin (CaN). Rj is a function of [Ca2+]i which increases during rapid pacing, and the level of connexion 43 (Cx43) phosphorylation. This study aims to investigate the association between CV, [Ca2+]i, and Rj and role of CaN as a mediator. Guinea pig left ventricular papillary muscles were superfused with control Tyrode’s solution at 37°C and point stimulated to generate propagated APs, recorded at known distances, d, by 3 M KCl-filled microelectrodes and conduction delay, t, recorded. Conduction velocity, CV, was calculated as d/t. Rj was measured from recordings of tissue impedance, Z (30 Hz- 300 kHz), in an oil-gap bath to constrain current flow to the intracellular space. Total and dephosphorylated Cx43 protein expression was measured by Western blot. [Ca2+]i was increased by: 1) rapid pacing (1 to 5Hz) and 2) low Na (29 mM Na) containing Tyrode’s solution. Data are expressed as mean±SD/SE, significance (p<0.05) by Student’s t-test or one-way ANOVA. Rapid pacing reduced CV from 47.4±2.1 to 33.7±2.9 cm/s (p<0.001; n=12). Similarly, low Na Tyrode slowed CV to 64.3±11.4% of control (p<0.05; n=4). Pretreatment with the calcineurin inhibitor, cyclosporine A (CysA; 10 µM) reversed the slowing in completely the slowing of CV at 5 Hz (p<0.001) and partially with low Na solution (80.0±5.7% of control; p<0.05; n=4). The latter intervention increased Rj (360±86.9 to 687±85; p<0.01; n=6) as well as the expression of dephosphorylated Cx43 (n=3). This was not accompanied by any change in the total Cx43 protein expression. Furthermore, the gap junction uncoupler, carbenoxolone (20µM; acts via dephosphorylating Cx43) slowed CV (70.9±1.3 to 59.2±3.1; p<0.05; n= 6) and increased Rj (393±51 to 502±80; p<0.05; n=6). In summary, slowing of CV, which was reversed by CysA, was mainly due to an increase in Rj. We propose this is due to calcineurin-induced dephosphorylation of the gap junction protein Cx43, due to raised [Ca2+]i
Where applicable, experiments conform with Society ethical requirements