Involvement of TRPC in the Slow Force Response Observed in Mouse Ventricular Myocytes

Abstract

When cardiac muscle is stretched persistently, the contractile force slowly increases over several minutes. This phenomenon is termed by a slow force response (SFR). The SFR is due to a stretch-induced increase in Ca2+ influx, however, underlying mechanism remains unclear. The transient receptor potential, canonical (TRPC) channels are mechano-sensitive, non-selective cation channels. We hypothesized that activation of TRPC channels is involved in stretch-induced increase in Ca2+ influx in SFR. To test this hypothesis, mouse cardiac myocytes were isolated with collagenase, and a pair of computer-controlled piezo-positioned carbon fibers was attached to each cell-end. Cells were paced at 1 Hz and superfused in normal Tyrode solution. Passive and active forces were calculated from carbon fiber bending. Stretch was applied by moving carbon fiber positions to achieve 3-10 % increase in sarcomere length and maintained for 200 to 300 seconds to obtain SFR. At the plateu of SFR, the contractile force increased to 115.7 ± 3.7% (n = 16) of the value immediately after the stretch. Inhibition of TRPC channel with BTP-2 (10 μM) significantly reduced the magnitude of SFR (103.5 ± 1.0%, n = 8). It has been reported that activation of angiotensin II type 1 (AT1) receptor induces Ca2+ influx via TRPC channels. To reveal whether TRPC channels are activated secondarily by AT1 receptor activated by strech, we tested the effect of Olmesartan, an AT1 receptor inhibitor, in SFR. Olmesartan (10 μM) also reduced the magnitude of SFR (101.1 ± 2.4%, n = 8). These results suggest that stretch-induced activation of AT1 receptors leads to increase in Ca2+ influx via TRPC channels in SFR.