M yocardial ischemia elicits an enhanced responsivity to alpha 1-adrenergic stimulation and a reversible increase in alpha 1-adrenergic receptor number. In adult cardiac myocytes, alpha 1-adrenergic receptor number increases two- to threefold after 10 min of hypoxia, an increase similar to that seen during ischemia in vivo. To determine whether this increase in alpha 1-adrenergic receptor number leads to an enhanced synthesis of inositol trisphosphate, the intracellular second messenger for the alpha 1-adrenergic receptor, the mass of inositol trisphosphate was quantified by a novel procedure developed in our laboratory that circumvents problems associated with using labeled precursors. The peak increases in inositol trisphosphate levels of three- to fourfold were measured after 30 s of norepinephrine stimulation and exhibited a 50% effective concentration (EC50) of 7.9 x 10(-8) M. Hypoxia produced a marked leftward shift in the dose-response curve for the production of inositol trisphosphate in response to norepinephrine stimulation (EC50 = 1.2 x 10(-8) M). Hypoxia also induced a 100-fold reduction in the concentration of norepinephrine required to elicit a threshold increase in inositol trisphosphate (10(-9) M), compared with control normoxic myocytes (10(-7) M). Thus, hypoxia, which increases alpha 1-adrenergic receptor density, also leads to an enhanced production of inositol trisphosphate and could account for the enhanced alpha 1-adrenergic responsivity in the ischemic heart in vivo, which is known to facilitate arrhythmogenesis.