Figure 8. Na⁺-Ca²⁺ exchange current is increased in CSQ mice.

A, Na⁺-Ca²⁺ exchange current was quantified as the Ni²⁺-blockable component (trace a – b) of the outward current generated on rapid removal of external Na⁺. Arrows indicate addition of 5 mmol l⁻¹ Ni²⁺. The holding potential was −40 mV. Note the current recovers to levels negative to resting values, as expected from higher cellular levels of Ca²⁺ following the large influx of Ca²⁺. B, mean densities of Na⁺-Ca²⁺ exchange currents measured as described in A were significantly higher in CSQ compared with control myocytes (** P < 0.01). Error bars are s.e.m. Control: 6 myocytes from 3 hearts; mean age, 138 days. CSQ: 6–10 myocytes from 3 hearts; mean age, 150 days.