Neural regulation of blood pressure in rainbow trout (Salmo gairdneri)

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Abstract
Mean dorsal aortic blood pressure (Pda) and heart rate were measured in free-swimming rainbow trout (Salmo gairdneri). The fish were swum in a water tunnel at 0.5 body lengths/s (control) and were exercised at 45-min intervals by a 5-min period of rapid swimming at 2 body lengths/s.Control Pda was 4.0 ± 0.11 kPa and control heart rate was 56 ± 2.1 beats/min. During rapid swimming Pda and heart rate increased by 15 and 13% respectively. α-Receptor blockade with phentolamine (2 mg/kg) or adrenergic transmitter release blockade with bretylium (10 mg/kg) prevented the exercise hypertension and converted it to a decrease of 5% (phentolamine) or 18% (bretylium). Exercise tachycardia was reduced slightly by each compound. Phentolamine decreased the control Pda by 5%. Acute bretylium treatment increased Pda by 39% but Pda in fish treated chronically with bretylium was 30% below control and decreased by a further 6% during rapid swimming. The acute effects of these and other antihypertensive compounds are discussed. It is concluded that systemic blood pressure in trout is controlled by tonically active adrenergic nerves acting on systemic vessels via α-adrenoreceptors. These nerves produce a selective increase in peripheral resistance during rapid swimming.