Research Doctorate - Doctor of Philosophy (PhD) / There is no consistency between studies of mammalian autonomic systems controlling the coronary circulation, particularly with respect to vagal cholinoceptor activity. Therefore, integrated autonomic control of regional coronary blood flow and conductance was investigated in chronically prepared awake sheep, and compared with data from this laboratory found in the awake dog. Coronary blood flow was measured simultaneously in the circumflex (Cx), right (R) and anterior descending (AD) beds by pulsed Doppler probes. Heart rate (HR) was controlled over the range of 100 to 180 beats/min by atrioventricular pacing. Baroreflex responses were evoked by elevating upper-body aortic pressure (Pa) using an external aortic occluder at HR of 150 and 180 beats/min. Specific agonists determined that effective populations of α-, β-adrenoceptors and cholinoceptors exist in all three coronary beds. Cholinoceptors (ch) and β-adrenoceptors (β) evoke vasodilatation, whereas α-adrenoceptors (α) evoke vasoconstriction. By contrast, electrical vagal stimulation constricts all 3 coronary beds, an effect blocked by methscopolamine. Pacing the heart upward raised baroreflex and metabolic activity and a differential rise in coronary conductance where Cx was greatest followed by R then AD. Block of ch, β and α alone, and together, at different HR revealed that, 1. in all 3 beds an underlying ch constrictor effect is balanced by the summed dilator effects of β, plus a probable α-enhanced vagal, vasointestinal peptide (VIP) interaction mechanism, 2. the net effects summate to enhance flow and conductance in Cx, but not in R and AD. At high HR, there is some waning of the neural mechanisms but raising and sustaining Pa at constant HR can recruit these mechanisms in each bed indicating ch, β and α/VIP, are baroreflex dependent even at the highest physiological HR. Therefore, differences do exist between species, for in particular baroreflex evoked cholinoceptor effects in the mammalian coronary circulation. They are vasoconstrictor in sheep, and vasodilator in the dog. These species differences specify successful alternatives in the evolution of reflex systems to meet the demand for coronary blood flow.
| Identifer | oai:union.ndltd.org:ADTP/280675 |
| Date | January 2009 |
| Creators | Hamut , Mutalip |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright 2009 Mutalip Hamut |
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