Previously, we have demonstrated the sinoatrial (SA) node contained an area lacking the gap junctional protein, connexin43 (Cx43), and that the area increased proportional to the animal’s age (Jones et al. 2001). Ca_v1.2 provides a depolarizing current during pacemaker activity in the SA node and reduced expression would produce a decline in SA node function. Now we have investigated changes in protein expression of the L-type calcium channel (Ca_v1.2) within the SA node during ageing.

Guinea-pigs were humanely killed by anaesthetic overdose and the heart removed. The SA node was dissected from the right atrium and 14 µm sections were dual labelled with anti-Cx43 (Chemicon, USA) and anti-Ca_v1.2 (Alomone, Israel) plus IgG conjugated to FITC/rhodamine and imaged by confocal laser-scanning microscopy. Data are shown as means ± S.E.M.

Expression of Ca_v1.2 protein was observed as punctate labelling around the outer membrane of atrial cells, but was absent from the centre of the SA node (Fig. 1). The area lacking Ca_v1.2 protein significantly increased from 2.06 ± 0.1 mm² at 1 month (n = 4), to 6.23 ± 0.4 mm² at 18 months (n = 3), to 7.45 ± 0.9 mm² at 26 months (n = 4) and 18.72 ± 2.2 mm² at 38 months (n = 3) (ANOVA, P < 0.0005). When the SA node preparation was maintained in oxygenated Tyrode solution at 37°C, an extracellular electrode was used to record the intrinsic heart rate, which decreased with age from 249 ± 13 beats min^-1 at 1 day to 177 ± 4.5 beats min^-1 at 1 month and 152 ± 5 beats min^-1 at 38 months (n = 5; ANOVA, P < 0.02). However, beating of the heart was stopped by 100 µM nifedipine at 1 day compared with 30 µM at 1 month and 10 µM at 38 months (n = 5; ANOVA, P < 0.005).

We conclude that Ca_v1.2 protein expression decreased in the SA node with age and this may affect the heart rate and sensitivity to nifedipine.

This work was supported by the British Heart Foundation.