Ca2+ Influx Through T- and L-Type Ca2+ Channels Have Different Effects on Myocyte Contractility and Induce Unique Cardiac Phenotypes

T-type Ca2+ channels (TTCCs) are expressed in the developing heart, are not present in the adult ventricle, and are reexpressed in cardiac diseases involving cardiac dysfunction and premature, arrhythmogenic death. The goal of this study was to determine the functional role of increased Ca2+ influx through reexpressed TTCCs in the adult heart. A mouse line with cardiac-specific, conditional expression of the alpha 1G-TTCC was used to increase Ca2+ influx through TTCCs. alpha 1G hearts had mild increases in contractility but no cardiac histopathology or premature death. This contrasts with the pathological phenotype of a previously studied mouse with increased Ca2+ influx through the L-type Ca2+ channel (LTCC) secondary to overexpression of its beta 2a subunit. Although alpha 1G and beta 2a myocytes had similar increases in Ca2+ influx, alpha 1G myocytes had smaller increases in contraction magnitude, and, unlike beta 2a myocytes, there were no increases in sarcoplasmic reticulum Ca2+ loading. Ca2+ influx through TTCCs also did not induce normal sarcoplasmic reticulum Ca2+ release. alpha 1G myocytes had changes in LTCC, SERCA2a, and phospholamban abundance, which appear to be adaptations that help maintain Ca2+ homeostasis. Immunostaining suggested that the majority of alpha 1G-TTCCs were on the surface membrane. Osmotic shock, which selectively eliminates T-tubules, induced a greater reduction in L-versus TTCC currents. These studies suggest that T- and LTCCs are in different portions of the sarcolemma (surface membrane versus T- tubules) and that Ca2+ influx through these channels induce different effects on myocyte contractility and lead to distinct cardiac phenotypes. (Circ Res. 2008; 103: 1109-1119.)