Figure 7. Effects of serine/threonine protein phosphatase inhibitors, okadaic acid and calyculin A, on I_to,fast in mouse left ventricular myocytes.

A, whole-cell currents were monitored continuously in the presence of 50 μm 4-AP (voltage-clamp protocols shown on the top, also see Fig. 3A for details). Top panel, I_to,fast was recorded every 1 min then normalized to the initial corresponding value at time 0 when cardiac myocytes were consecutively exposed to isosmotic, isosmotic + OA (100 nm), hyposmotic + OA (100 nm), and hyposmotic solutions. Changes in perfusion solutions were started when the changes in current amplitude reached the steady-state level. a–d, the representative superimposed current traces (upper) and the prepulse-sensitive difference current (I_to,fast) traces (lower) recorded from the same cell as shown in the top panel at the time points indicated by the arrows. B, changes in mean peak I_to,fast current densities (•, y-axis on the left) and cell volume (•, y-axis on the right) recorded when cells were consecutively exposed to isosmotic (Iso), isosmotic +100 nm OA (Iso + OA), hyposmotic +100 nm OA (Hypo + OA), and hyposmotic (Hypo) solutions (n = 6). Currents were obtained using the protocols described in panel A (*P < 0.05 when compared with cell volume under control (Iso) conditions; ^#P < 0.05 when compared with peak current density under control (Iso) conditions). C, changes in mean peak I_to,fast current densities (•) and cell volume (•) recorded from cardiac myocytes when they were consecutively exposed to isosmotic (Iso), isosmotic + 20 nm CA (Iso + CA), hyposmotic + 20 nm CA (Hypo + CA), and hyposmotic (Hypo) solutions (n = 5, *P < 0.05 when compared with cell volume under control (Iso) conditions; ^#P < 0.05, ^##P < 0.01 when compared with peak current density under control (Iso) conditions).