Previous studies have shown developmental changes in cardiac L-type Ca2+ channel function. The β subunit of the L-type Ca2+ channel plays an important role in regulating channel activity. Therefore, we hypothesized that developmental changes in cardiac Ca2+ channel function may reflect age-related changes in β subunit steady-state mRNA abundance. Total cellular RNA was prepared from fetal (F: 28d gestation), neonatal (D1, D5), juvenile (D15, D30) and adult (AD: 3 mo) rabbit ventricles. Northern analysis of size-fractionated total cellular RNA (10 μg) was performed with a radiolabelled cDNA fragment encoding the β2 subunit of the rat Ca2+ channel. The probe hybridized to two mRNA species with sizes of 6 and 4.7 kb. We found that the 6 kb band was not apparent in fetal or neonatal tissue but increased in abundance from day 15 to adult. Densitometry data for each blot were normalized to the amplitude of the 4.7 kb band in adult tissue. The relative hybridization intensity of the 6 kb band was: F: 8.5±3.5 (4), D1: 17.3 (2), D5: 23.8±2.5 (3), D15: 75.2(2), D30: 83.9±18.5 (3), AD: 78.7±9.0 (4). (values represent mean±SEM(n), p<0.01 by ANOVA). In contrast, the 4.7 kb band exhibited little change in abundance with development: F: 84.7±25.1(4), D1: 80.8 (2), D5: 89.3±8.1 (3), D15: 106.3 (2), D30: 110.9±13.7 (3), AD: 100 (4). These results suggest that mRNA levels of the L-type calcium channel β subunit are differentially regulated in developing rabbit heart. Differential expression of β subunit isoform may explain the previously reported age-related changes in calcium channel activity.
