Chronically catheterized specimens of the toad Bufo marinus (L.) were given single bolus injections, periodic bolus injections, and continuous infusions ( via peristaltic pump) of the neuropeptide arginine vasotocin (AVT). Urine flow and glomerular filtration rates (GFR) were monitored to quantify the antidiuretic response of the toad to AVT under these conditions. The response to single bolus injections was dissipated after 1h, despite the continued presence of AVT, whereas continuous infusion of AVT at levels as high as 100 ng kg −1 was ineffective in lowering flow rates, suggesting that pulsatile Increases in the hormone might be required to effect a more prolonged response. In toads which were ‘primed’ with AVT via continuous infusion with the pump, additional pulses of AVT were able to lower flow rates for lh, with sensitivity to the hormone decreasing with an increase in primed concentration of AVT. Pulses of AVT given to unprimed toads significantly lowered flow for over 2h. This study is the first to show that phasic bursts of AVT are more effective in regulating the renal response of the toad than are continuous, nonpulsatile infusions.
1. Oxygen and carbon dioxide tensions were determined in the lungs and in blood from the dorsal aorta, pulmonary vein, pulmonary artery and inferior vena cava in the intact, free swimming, Amphiuma . At 15° C this animal was submerged for a large part of the time and surfaced briefly to breathe at variable time intervals, the mean period being 45 min. 2. Oxygen tensions in the lungs and in all blood vessels oscillated with the breathing cycles, falling gradually during the period of submersion and rising rapidly after the animal breathed. The absolute level of oxygen tension did not appear to constitute the effective signal beginning or ending a series of breathing movements. 3. A small oxygen gradient existed between lungs and blood in the pulmonary vein immediately after a breath. The gradient increased in size as an animal remained submerged due, it is suggested, to lung vasoconstriction increasing the transfer factor. 4. Blood in the dorsal aorta had a lower oxygen tension than that in the pulmonary vein. A right-to-left shunt occurred as blood moved through the heart. The degree of shunting increased as the animal remained submerged and pulmonary vasoconstriction occurred. Left-to-right shunt was relatively insignificant since oxygen tensions in the inferior vena cava and pulmonary artery were very similar. 5. Carbon dioxide tensions were relatively constant during the breathing-diving cycle since Amphiuma removed almost all of this gas through the skin.