Carvedilol increases blood pressure response to phenylephrine infusion in heart failure subjects with systolic dysfunction: evidence of improved vascular alpha1-adrenoreceptor signal transduction.

INTRODUCTION: Alpha(1)-adrenergic receptor (alpha(1)-AR) stimulation produces smooth muscle contraction, vasoconstriction, and myocyte hypertrophy, suggesting a potential therapeutic role for alpha(1)-AR antagonists to reduce cardiac workload and myocardial hypertrophy. Preliminary reports suggest that vascular alpha(1)-ARs are desensitized in heart failure (HF) in a manner similar to myocardial beta(1)-ARs. We examined alpha(1)-AR signal transduction by repeat phenylephrine (PE) infusions in patients with HF receiving chronic carvedilol therapy.
METHODS: Twelve subjects with HF not currently receiving beta-blockers were up-titrated to maximum tolerable doses of carvedilol. Subjects underwent alpha(1)-AR stimulation testing at study baseline, 2 weeks after each dose titration, and 6 months after maintenance of maximum carvedilol dose. Phenylephrine infusions began at 0.5 microg kg(-1) min(-1), with dose titrations every 10 minutes, to a maximum of 5 microg kg(-1) min(-1). Phenylephrine dose response was evaluated by the PE rate required to elicit a 20 mm Hg increase in systolic blood pressure (BP), designated PS(20).
RESULTS: All doses of carvedilol significantly reduced preinfusion measures of heart rate, systolic BP, diastolic BP, and mean arterial pressure. However, carvedilol also produced a paradoxical trend toward PS(20) reduction (indicating increased PE response) that reached significance at the completion of carvedilol dose titration (PS(20) ratio vs baseline = 0.78; P < .001). All effects were maintained over a 6-month treatment period with no evidence of tolerance.
CONCLUSIONS: Increasing BP response to PE infusion suggests improvement in vascular alpha(1)-AR signal transduction with chronic carvedilol therapy. This effect is evident despite no detectable tolerance to preinfusion BP reductions. The varying affinities of alpha(1)-AR subtypes for carvedilol and PE may have contributed to this finding.