Endothelin-1 (ET-1) acts via multiple signalling pathways to modulate the inotropic status of the heart and can cause arryhthmias. This study compared the effects of ET-1 on the patterns and kinetics of calcium signalling in atrial and ventricular myocytes, and examined the role of InsP3 in changes evoked by ET-1.
Myocytes were enzymatically isolated from rats killed by cervical dislocation in accordance with Schedule 1 Home Office regulations. Cells were electrically paced at 0.3 Hz and calcium changes were monitored using indo-1. Experiments were conducted at room temperature. All statistics represent means ± S.E.M.
In response to 100 nM ET-1, atrial myocytes displayed a brief (~3 min) phase of negative inotropy followed by a positive inotropic response that peaked at 254 ± 17 % relative to control amplitude responses (n = 7 cells) after 16 min exposure. In contrast, ventricular myocytes exhibited no negative inotropic phase, and achieved a maximum inotropic response of 193 ± 22 % after 21 min (n = 15 cells). ET-1 stimulation caused substantial numbers of spontaneous calcium transients (SCTs) in atrial and ventricular cells. These events were rare in control recordings. For atrial cells, SCTs began at the onset of the positive inotropic response, after 5 min of ET-1 simulation. They progressively increased in frequency and amplitude such that at 25 min of ET-1 stimulation an average of 19 ± 1.4 SCTs per 30 s sample period (n = 7 cells). For ventricular myocytes, SCTs arose within 1 min of ET-1 stimulation and reached a maximum of 12 ± 2.2 SCTs (n = 15 cells).
Stimulation with a membrane-permeant form of InsP3 (InsP3 ester; 10 µM) caused a modest positive inotropic response in both atrial and ventricular myocytes. The most prominent effect of the InsP3 ester was to provoke the occurrence of SCTs. Consistent with the greater InsP3 receptor expression in atrial myocytes, the InsP3 ester-evoked SCTs were more frequent in the atrial cells (25 ± 7.5 events per 30 s sample period; n = 4 cells) than in ventricular cells (6 ± 2 events per 30 s sample period; n = 14 cells). In both cell types, the ET-1 and InsP3 ester-evoked SCTs were inhibited by 2-aminoethoxydiphenyl borate (2 µM). These data indicate subtly different effects of ET-1 on calcium signalling in atrial and ventricular cells. Furthermore, InsP3 may underlie some of the pro-arrythmogenic actions of ET-1, but does not significantly influence the inotropic status of these cells.