Sarcoplasmic reticulum Ca²⁺ load in human heart failure

Abstract

Excitation-contraction coupling and intracellular Ca²⁺ homeostasis are altered in heart failure. We tested the hypothesis that these changes are related to disturbed Ca²⁺ handling of the sarcoplasmic reticulum (SR).

Isolated, electrically stimulated trabeculae were obtained from end-stage failing (NYHA IV) and nonfailing human hearts. Isometric twitch tension, intracellular Ca²⁺ transients (aequorin method) and SR Ca²⁺ content (rapid cooling contractures) were assessed under basal conditions (1 Hz, 37 °C) as well as after stepwise increasing rest intervals from 2 – 240 s (post-rest contractions). Protein expression of SERCA2a and phospholamban (Western blot) was assessed in a subset of failing trabeculae. In addition, the effects of SERCA1 overexpression on contractile function of isolated myocytes was tested.

On average, post-rest twitch tension continuously increased with increasing rest intervals in nonfailing, but declined with rest intervals longer than 15s in failing myocardium. The rest-dependent contractile changes were accompanied by parallel changes in intracellular Ca²⁺ transients. Failing trabeculae (n = 40) were grouped (group A: post-rest potentiation (force of contraction > pre-rest twitch force) after 120s rest interval; group B: post-rest decay (force of contraction < pre-rest twitch force) after 120 s rest interval), and post-rest contractile function was related to SERCA2a and PLB expression. While PLB protein expression was not different, SERCA2a protein expression as well as SERCA2a/PLB ratio was significantly higher in group A vs. group B. Transfection of SERCA1 increased shortening amplitude and enhanced relaxation kinetics in failing human myocytes.

In conclusion, SR Ca²⁺ handling is severely altered in human heart failure. Reduced SR Ca²⁺ release is due to diminished SR Ca²⁺ content directly related to a depressed expression of SERCA2a protein. Enhancing SERCA function or expression may improve SR Ca²⁺ handling in failing human myocardium.