PERIPHERAL BETA-ADRENERGIC-BLOCKADE MODIFIES AIRPUFF STARTLE-INDUCED HEART-RATE RESPONSES

Prior studies showed that repeated airpuff startle-reaction stimuli ap plied to normotensive inbred Wistar-Kyoto rats bred in La Jolla or Spr ague-Dawley rats elicit presser responses on all trials except trial-d ependent bradycardia or tachycardia. However, hypertensive inbred spon taneously hypertensive rats bred in La Jolla exhibited no bradycardia. Peripheral methylatropine blocked bradycardia and unmasked tachycardi a, which implies concurrent autonomic discharges on early trials. As s hown here, Vendor inbred Wistar-Kyoto rats from Charles River Laborato ries (WKYCR), fail to show bradycardia. Because stress-induced parasym pathetic responses are important to understanding arrhythmogenesis, we tested whether (WKYCR) and inbred spontaneously hypertensive rats fro m Charles River Laboratories (SHR(CR)) exhibit any parasympathetic act ivation by blunting sympathetic chronotropic responses with cardiosele ctive beta-adrenoceptor antagonists. WKYCR or SHR(CR) were pretreated with either nonselective propranolol, beta(1)-selective metoprolol, be ta(1)-selective celiprolol (with beta(2)-receptor agonist activity) or ICI 118,551, a selective beta(2)-receptor antagonist. Neither propran olol nor metoprolol affected resting HR in WKYCR, but both decreased H R in SHR(CR), whereas celiprolol raised resting HR only in WKYCR. Alth ough control WKYCR nor SHR(CR) exhibited bradycardia, bradycardia was unmasked in both by all beta(1)-selective agents but not by ICI 118,55 1. However, ICI 118,551 reduced tachycardia responses over all trials in WKYCR, which suggests beta(2)-adrenoceptor involvement in the stres s-induced tachycardia. Significant cardiac contributions to the presse r responses in both WKYCR and SHR(CR) were found. We conclude that the mild airpuff startle-reaction stress stimulus elicits simultaneous pa rasympathetic and sympathetic discharge on early trials in both normot ensive and hypertensive rats, that both beta(1)- and beta(2)-adrenocep tors are involved in HR responses and that the airpuff induces simulta neous sympathetic and parasympathetic responses, the latter having a p otential cardioprotective role.