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Role of Beta-Adrenoceptors in Cooling-Evoked Hemodynamic Perturbations of Rats: Investigation by Spectral Analysis

Abstract

Yia-Ping Liu, Yi-Hsien Lin, Yu-Chun Chen, Po-Lei Lee and Che-Se Tung

Aim: Rapid immersion of a rat’s limbs into 4°C water, a model of cooling stress (CS), can elicit hemodynamic perturbations (CEHP). We have reported that CEHP is highly relevant to the sympathetic activation. This study identifies the role of β-adrenoceptors in CEHP.

Method: Conscious rats were pretreated with the β-adrenoceptor blockade propranolol, (PRO)-only, or following the removal of sympathetic influences using hexamethonium (HEX) or guanethidine (GUA), and then they were subjected to a 10-min CS trial. Cardiovascular indices were monitored via an implanted telemetric device throughout the experimental course. The analyses included measurements of systolic blood pressure (SBP); heart rate (HR); cardiovascular variability (BPV; HRV); spectral coherence at very-low, low, and high frequency regions (VLF: 0.02-0.2 Hz, LF: 0.2-0.6 Hz, and HF: 0.6-3.0 Hz); total power (TP); and dicrotic notch (Dn).

Results: Compared with the vehicle control under the resting (PreCS) and CS conditions respectively, the PROonly (a) increased the powers for VLFBPV, LFBPV, HFBPV, HFHRV, and TPBPV but decreased the power for VLFHRV, the LF/ HF ratio, the Dn under PreCS; (b) increased the powers for LFHRV, HFHRV, and TPHRV, decreased the powers for LFBPV, HFBPV, and TPBPV, the LF/HF ratio, and the Dn, and converted the original negative correlation into positive correlation for VLFHRV with VLFBPV under CS; and (c) weakened the spectral coherence at all frequency regions between BPV and HRV throughout the experimental course. Compared with the control vehicle under PreCS and CS, there were more decreases of SBP (under CS) and HR (under PreCS and CS) after the GUA+PRO than the other interventions (PROonly and/or HEX+PRO). In addition, the effect on spectral powers of the PRO-only was generally altered when the rats were pretreated with HEX or GUA throughout the experimental course.

Significance: Our findings suggest that PRO may exert vascular effects which are dependent on the sympathetic vasodilator tone. Intact sympathetic efferent pathways are required for the inhibition of CEHP by PRO. Besides, the effects of HEX+PRO versus GUA+PRO indicate a functional role of adrenal medulla to release epinephrine to react the cooling stress.