The main features of heart failure(HF) pathophysiology are the impaired contraction of ventricular muscle and arrhythmogenic risk. Loss or disorganization of the network of transverse tubules has turned out as an early feature of cardiomyocyte remodeling in heart failure. However, the impact of t-tubular remodelling on the mechanics or on the arrhythmogenic potential of the myocardium was never before investigated. We first developed and validated a novel method to achieve acute disconnection of t-tubules from the surface sarcolemma in ventricular trabeculae with formamide-induced osmotic shock and evaluated the effects of detubulation on force development. We then introduced a novel optical technique to simultaneously record electrical activity from multiple small regions of the t-tubular network and surface sarcolemma employing voltage sensitive membrane dyes. The electrical function of t-tubules was evaluated on control myocytes and cells from rat with post-ischemic heart failure induced by coronary ligation. Acute loss of t-tubules in trabeculae significantly reduces myocardial force and impairs the ability to increase contraction in response to high frequency. Heart failure myocytes show a reduced t-tubular density with patchy areas devoid of t-tubules and display smaller global calcium transients with areas of delayed and incomplete calcium release. Local optical recordings show that action potentials occur simultaneously throughout the whole membrane in control myocytes; however, in HF, part of remodelled t-tubules does not show a regular electrical activity and often display local spontaneous depolarizations. Altered t-tubular electrical function is an additional mechanism besides orphaned ryanodine receptors contributing to reduced force in HF and spontaneous activity of altered tubules may produce arrhythmias.