Fig. 3.

Release of calcium from the ryanodine receptors (RyR) of the sarcoplasmic reticulum is essential for the hypoxic induction of myoglobin. Pharmaceutical stimulation of the different intracellular calcium pools [caffeine→RyR, KCl→L-type channel, and uridine 5′-triphosphate (UTP)→inositol 1,4,5-trisphosphate (IP₃) receptors] indicated that the hypoxic induction in myoglobin was dependent on calcium release specifically by the ryanodine receptors of the sarcoplasmic reticulum. Stimulation of normoxic 4-day myotubes with caffeine or KCl recapitulated ∼75% or 60% of the hypoxic stimulation, respectively. In contrast, stimulation of the IP₃ receptor pool of calcium from the endoplasmic reticulum with UTP inhibited the transcription of myoglobin (n = 6, P = 0.02). *Significant difference between normoxic and hypoxic conditions. **Significant difference between UTP stimulation and all other conditions.