Integration of baroreflex and autoregulation control of bronchial blood flow in awake dogs

Abstract

Baroreflex control of the bronchial circulation is unresolved. Early studies suggested that baroreflexes dilate or have no effect, but recent studies in awake dogs suggested baroreflexes did not normally engage tonic vasoconstrictor efferents but during excitement systemic pressure rises may also trigger local sensory-motor dilator reflexes. We examined the postulate that bronchial flow is normally regulated at rest during controlled changes in pressure gradient (Pg) by integration of tonic autonomic activity with autoregulation. Twelve greyhounds were instrumented under general anaesthesia by surgical implantation of pulsed Doppler flow transducers on the right bronchial artery (BA). After recovery baroreflex effects were evoked by raising and lowering aortic pressure using a lower thoracic aortic balloon in 11 animals, and in six of these after cholinoceptor plus adrenoceptor blockade. The right BA bed showed pressure-passive responses and the time-dependent bronchial bed effects in the autonomically intact state (INT) were largely similar to those in the blocked state (TAB). When results were replotted as pressure-flow relationships and analysed using covariance, the regression line over the pressure range 70-135 mmHg for TAB demonstrated a significant slope (P < 0.05), a linear regression elevated 120% (P = 0.006) above and parallel to INT (Y(Q) = 0.034 + 0.00033(X(Pg) - 104.6). The regression fell on the line of equal proportional change. Baroreflexes do not functionally engage the autonomic outflow to the bronchial circulation. Under controlled conditions of systemic Pg change, the bronchial circulation is normally controlled by the integration of resting autonomic tone, myogenic autoregulation and pressure-passive effects.