Although it is well established that there is heterogeneity in the electrophysiological characteristics of single myocytes across the left ventricular free wall (LV), little is known about how these differences may influence excitation-contraction coupling (E-C C) within the heart.

E-C C was investigated in myocytes isolated from basal sub-endocardial (ENDO), mid-myocardial (MID) and sub-epicardial (EPI) regions of the guinea-pig LV. Animals were killed humanely. Membrane current and unloaded cell shortening (UCS) were measured using the whole-cell patch-clamp and video-edge detection techniques at 35 ± 1 °C. I_{Ca, L} and UCS were elicited by 300 ms step depolarisations to test potentials ranging between -50 and +80 mV. Sarcoplasmic reticulum (SR) Ca²⁺ load was assessed by integration of the caffeine-induced current in the presence of carboxyeosin (20 mM). The Na-Ca exchanger current (I_NCX) was elicited by a descending ramp protocol (+80 to -120 mV) and assessed as the Ni⁺-sensitive current. SR Ca-ATPase (SERCA2a), phospholamban (PLB) and NCX protein levels were determined using quantitative Western blot techniques and expressed relative to GAPDH. Data are presented as means ± S.E.M. (n), ANOVA significance level P < 0.05.

Peak amplitude of UCS was greater in EPI compared with ENDO myocytes (P < 0.05, Fig. 1A). The maximal rate of both contraction and relaxation was faster in EPI than in ENDO myocytes (P < 0.05). Although I_Ca,L density (Fig. 1B) and myofilament sensitivity were similar between the regions, SR Ca²⁺ load was significantly greater in EPI compared with ENDO myocytes (P < 0.002, Fig. 2A). In the presence of carboxyeosin, the integral of the caffeine-induced inward current was significantly increased in EPI but unaltered in ENDO myocytes (P < 0.05, Fig. 2A), suggesting that the contribution made by the sarcolemmal Ca-ATPase to Ca²⁺ extrusion was greater in EPI than in ENDO myocytes.

SERCA2a protein level was greater in EPI compared with ENDO myocytes, but PLB was greater in ENDO compared with EPI myocytes. The resultant reduction in the PLB : SERCA2a ratio (a useful indicator of contractility and relaxation) in EPI compared with ENDO myocytes is consistent with the faster kinetics in EPI myocytes.

These data show that contraction amplitude is greater and relaxation faster in EPI than ENDO myocytes. This is in part attributable to a greater SR Ca²⁺ load and a reduced PLB:SERCA2a ratio in EPI myocytes. In addition, I_Na-Ca density and NCX protein levels were greater in ENDO myocytes, which may underlie a greater propensity for arrhythmias seen in ENDO myocytes.

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