An Investigation of the Nature of the Neural and Local Mechanisms That Regulate Vascular Functioning

1. In the pithed rat, stimulation of the autonomic outflow at different levels between C1 and L3 produced characteristic increases in blood pressure and heart rate. Stimulation between C6 and T1 and between T2 and T4 produced large increases in heart rate with only small increases in blood pressure. Stimulation at T8-9 produced the largest increase in blood pressure and a large increase in heart rate. 2. Spinal nerve stimulation at T8-9 characteristically produced pressor responses during stimulation and a secondary, pressor response that persisted after stimulation ceased. These responses were frequency- dependent so that the response at 20 Hz (supramaximal voltage, 0.4 ms pulse-width, 100 pulses) was larger than that at 2 Hz (supramaximal voltage, 0.4 ms pulse-width, 100 pulses). 3. The pressor response to stimulation at T8-9 to 2 Hz and 20 Hz probably resulted from stimulation of nerve fibres to blood vessels and also to the adrenal glands, which contributed both to the component of the pressor response that occurred during stimulation and to the secondary component that persisted after stimulation ceased. This was indicated by experiments in which adrenalectomy reduced the first component of the pressor response that occurred during stimulation at T8-9 and abolished the second component that persisted after stimulation. 4. The effect of propranolol (2 mg kg-1) on both components of the pressor response to spinal nerve stimulation at T8-9 was complex and dependent on the basal blood pressure of the pithed rat. In most pithed rats, propranolol enhanced both components of the pressor response to nerve stimulation. This was most marked at 2 Hz and most obvious in the secondary poststimulation component, which probably resulted from adrenaline released from the adrenals. 5. In some experiments where the blood pressure was low (