Fig. 5.

SR Ca²⁺ stores. SR Ca²⁺ stores did not increase with rising pacing frequencies (1–6 Hz) in NOS1^–/– myocytes, as it did in WT and NOS3^–/– myocytes (*, P < 0.05 vs. 1-Hz baseline; †, P < 0.05, NOS1^–/– vs. WT and NOS3^–/–). Importantly, allopurinol (10^–4 M, open symbols) had no effect on SR Ca²⁺ stores in NOS1^–/–, NOS3^–/–, or WT myocytes.