TRPC channels, an overarching Ca(2+) paradigm in the developing heart.

It is well established that in the adult mammalian heart, small amounts of Ca2+ entry via voltage-dependent sarcolemmal Cav1.2 L-type Ca2+ channels can trigger the release of a significant amount of Ca2+ from the sarcoplasmic reticulum (SR) that is followed by muscle contraction, a process known as excitation–contraction (EC) coupling. However, for the developing mammalian and avian hearts, earlier studies proposed that EC coupling relies essentially on transsarcolemmal influx of Ca2+ through L-type Ca2+ channels, mainly Cav1.2, and that the contribution of the SR is minimal. In the embryonic chick heart, Ca2+ channel density is maximal at 3 days and decreases at 17 days, whereas Ca2+ channel density is low in the foetal heart and increases with development in guinea-pig, rabbit, and human, consistent with the need for additional Ca2+ entry through other sarcolemmal voltage-dependent proteins such as the T-type Ca2+ channel, the Cav1.3 L-type Ca2+ channel, and the Na/Ca2+ exchanger (NCX). 8 Other candidate channels suspected to provide Ca2+ entry into the immature myocyte have been associated with voltage-independent channels, canonical transient receptor potential (TRPC) channels. TRPC channels are composed of four subunits with each containing six membrane-spanning regions lacking the voltage sensors in the fourth domain (thus voltage-independent). TRPC channels possess multiple regulatory and protein interaction sites, a property that allows them to associate and form homotetramers or heterotetramers. TRPC channels are activated following stimulation of receptors coupled to phospholipase C pathways through inositol trisphosphate and diacylglycerol (receptor-operated channels or ROCs), by depletion of internal calcium stores (store-operated channels or SOCs), and by membrane stretching (stretch-activated channels). The functional role of TRPC channels in the immature heart is just emerging. In this context, Sabourin et al. demonstrated that transarcolemmal Ca2+ entry via TRPC channels regulates not only contractility but also pacemaker activity and conduction in hearts from 4-day-old chick embryos. Sabourin et al. first showed that the transcript and proteins of TRPC 1, 3, 4, 5, 6, and 7 isoforms, but not TRPC 2, are present in the heart of the chick embryo. The expression of these multiple TRPC isoforms in the same embryonic myocyte suggests unique and discrete roles. Next, Sabourin et al. used co-immunoprecipitation technique to demonstrate that TRPC channels can form a macromolecular complex with the Ca1.2 a1C subunit of the L-type Ca2+ channel, in line with previous studies showing that TRPC channels can also form macromolecules with NCX, Na+/K+ pump, and SERCA pump. Although it is not clear what are the functional consequences of these co-associations, the ability of TRPC channels to interact with multiple Ca2+ modulators may create a microenvironment facilitating the fine tuning of Ca2+ homeostasis and EC-coupling. The observation that inhibition of TRPC channels by SKF, a blocker of TRPC channels, resulted in a decreased spontaneous beating rate in the whole embryonic heart and isolated atria suggests a potential role of TRPC channels in the pacemaker activity; however, the expression of TRPC channels in the sino-atrial (SA) node and the atrioventricular (AV) node of the embryonic chick heart was not investigated. Because inhibition of TRPC channels with SKF prevented tachycardia induced by SR Ca2+-ATPase blockade, it was suggested that substantial Ca2+ influx generating inward currents could be induced by SR store depletion, a characteristic of SOC activity. SOCs formed by TRPC channels have been recently proposed to provide a pacemaker current in the mouse SA node, thus supporting the concept that in the embryonic chick heart, TRPC channels may also contribute to the pacemaker activity as SOCs. However, the extent to which SOCs participate along with other channels to control the diastolic depolarization remains unknown. Inhibition of TRPC channels also resulted in conduction abnormalities such as firstand second-degree AV block, invoking a potential functional link to Cx43 as demonstrated in endothelial cells. SKF only partially diminished the myocardial shortening, indicating that TRPC channels may have only a complementary role in the control of Ca2+ entry and Ca2+ homeostasis. Collectively, the proposal that TRPC channels can act as SOCs and form a functional unit with the Cav1.2 L-type Ca2+ channel, thereby generating inward current that will contribute to the diastolic depolarization during the pacemaker phase, is intriguing. SOCs