The mechanisms by which -adrenergic stimulation of the heart in-vivo can cause contractile dysfunction are not well understood. We hypothesized that -adrenergic mediated contractile dysfunction is mediated through protein kinase C phosphorylation of troponin I, which in in-vitro experiments has been shown to reduce actomyosin MgATPase activity. We studied pressure-volume loops in transgenic mice expressing mutant troponin I lacking protein kinase C phosphorylation sites, and hypothesized altered responses to phenylephrine. As anesthesia agents can produce markedly different effects on contractility, we studied 2 agents: avertin and -chloralose-urethane. With -chloralose-urethane, at baseline, there were no contractile abnormalities in the troponin I mutants. Phenylephrine produced a 50% reduction in end-systolic elastance in wild type controls, though a 9% increase in troponin I mutants (P<0.05). Avertin was associated with reduced contractility compared to -chloralose-urethane. Avertin anesthesia, at baseline, produced a reduction in end-systolic elastance by 31% in the troponin I mutants compared to wild type (P<0.05), and this resulted in further marked systolic and diastolic dysfunction with phenylephrine in the troponin I mutants. Dobutamine produced no significant difference in the contractile phenotype of the transgenic mice with either anesthetic regimen. In conclusion, these data (-chloralose-urethane) demonstrate that -adrenergic mediated force reduction is mediated through troponin I protein kinase C phosphorylation. -Adrenergic responses are not mediated through this pathway. Altering the myofilament force-calcium relationship may result in in-vivo increased sensitivity to negative inotropy. Thus, choice of a negative inotropic anesthetic agent (avertin) with phenylephrine can lead to profound contractile dysfunction.