Changes in blood pressure during isometric contractions to fatigue in the cat after brain stem lesions: effects of clonidine

• Published in: Cardiovascular research 1990-10, Vol.24 (10), p.821-833
• Main Authors: Williams, Carole A, Roberts, Jon R, Freels, Douglas B
• Format: Article
• Language: English
• Subjects: Animals; Blood Pressure - physiology; Brain Stem - drug effects; Brain Stem - physiology; Cats; clonidine; Clonidine - pharmacology; Female; Hindlimb; Injections; Isometric Contraction - physiology; isometric contractions; Male; Medulla Oblongata - physiology; muscle pressor response; Periaqueductal Gray - physiology; periaqueductal grey matter; Raphe Nuclei - physiology; ventrolateral medulla
• ISSN: 0008-6363; 1755-3245
• DOI: 10.1093/cvr/24.10.821

Study objective – The aim was to determine whether areas in the periaqueductal grey matter, medial dorsal raphé, or ventrolateral medulla might be involved with the integration of blood pressure and heart rate during isometric exercise. Design – Cats were anaesthetised with α chloralose (75 mg·kg−1) and catheters inserted into the right jugular vein and carotid artery. Isometric contractions were generated using a microprocessor controlled stimulator and sleeve electrode around the tibial nerve. Bilateral lesions were made in the dorsal periaqueductal grey matter (P1.0, LR 2.0, HD + 1.5 mm) or two sites in the ventrolateral medulla (P12.0, RL 2.0, HD -10 mm; or P12.0, RL 2.0, HD -8.5 mm). Lesions were also made in the medial dorsal raphé nuclei (P1.0, RL 0.0, HD +1.5 mm). Clonidine was injected into the cerebral aqueduct to determine whether it would exert an antipressor effect during muscle contraction after the lesions were made. Only one site of lesion was made in a group of animals. Bilateral injections of clonidine (250 ng in 0.5 μl) were made into the intact ventrolateral medulla (P11.5, RL 4.0, HD -8.5 mm) to explore its role further. Fatiguing contractions were performed before and after the lesions were made, or clonidine was injected, and changes in arterial blood pressure and heart rate were measured. Verification of the lesion sites or the microinjection sites, and the extent of the lesion or spread of the clonidine, was made from histological examination of brain tissue after each experiment. Experimental material – Adult cats of either sex, n = 20, weight 2.4 (SD 0.4) kg, were used. Measurements and main results – Fatiguing isometric contractions in control conditions caused mean arterial pressure to increase by 45-50 mm Hg and heart rates by 20-25 beats·min−1. Bilateral lesions in the dorsal periaqueductal grey matter did not alter resting mean arterial pressure but attenuated the pressor response during contractions. Injections of clonidine into the cerebral aqueduct had no further antipressor effects after the lesions. Lesions of the medial dorsal raphé nuclei or injections of clonidine into the intact medial dorsal raphé nuclei did not affect the pressor response to fatiguing isometric contractions. Injections of clonidine into the intact ventrolateral medulla eliminated the pressor response to isometric contractions. Bilateral lesions of the ventrolateral medulla near the rostral lateral border of the inferior olivary tract nuclei (P12.0