RESUMO
Prior studies showed that repeated airpuff startle-reaction stimuli applied to normotensive inbred Wistar-Kyoto rats bred in La Jolla or Sprague-Dawley rats elicit pressor responses on all trials except trial-dependent bradycardia or tachycardia. However, hypertensive inbred spontaneously hypertensive rats bred in La Jolla exhibited no bradycardia. Peripheral methylatropine blocked bradycardia and unmasked tachycardia, which implies concurrent autonomic discharges on early trials. As shown here, vendor inbred Wistar-Kyoto rats from Charles River Laboratories (WKYCR) fail to show bradycardia. Because stress-induced parasympathetic responses are important to understanding arrhythmogenesis, we tested whether (WKYCR) and inbred spontaneously hypertensive rats from Charles River Laboratories (SHRCR) exhibit any parasympathetic activation by blunting sympathetic chronotropic responses with cardioselective beta-adrenoceptor antagonists. WKYCR or SHRCR were pretreated with either nonselective propranolol, beta 1-selective metoprolol, beta 1-selective celiprolol (with beta 2-receptor agonist activity) or ICI 118,551, a selective beta 2-receptor antagonist. Neither propranolol nor metoprolol affected resting HR in WKYCR, but both decreased HR in SHRCR, whereas celiprolol raised resting HR only in WKYCR. Although control WKYCR nor SHRCR exhibited bradycardia, bradycardia was unmasked in both by all beta 1-selective agents but not by ICI 118,551. However, ICI 118, 551 reduced tachycardia responses over all trials in WKYCR, which suggests beta 2-adrenoceptor involvement in the stress-induced tachycardia. Significant cardiac contributions to the pressor responses in both WKYCR and SHRCR were found.(ABSTRACT TRUNCATED AT 250 WORDS)
