The nature of atrial receptors responsible for the increase in urine flow caused by distension of the left atrium in the dog.

In dogs anaesthetized with alpha-chloralose the effects of distension of a large balloon in the lumen of the left atrium on the discharge of action potentials in vagal fibres and on urine flow were studied with the cervical vagi cooled at 18 and 12 degrees C. Distension of the balloon in seven dogs resulted in an increase in urine flow. Cooling the cervical vagi to 18 degrees C reduced the response to 70% of that obtained at 37 degrees C, and at 12 degrees C the response was reduced to 28%. In a second group of dogs, the effect of distension of the balloon on atrial receptors which discharged into myelinated nerve fibres of the vagi, i.e. Paintal type A and type B receptors, was examined. The increase in the activity of these receptors was reduced to 68% when the vagus nerve was cooled to 18 degrees C and was reduced further to 25% at 12 degrees C., In a third group of dogs, the effect of distension of the balloon on receptors which discharged into non-myelinated nerve fibres in the vagi was examined. Cooling of the cervical vagi also reduced the evoked increase in activity in these fibres; this reduction in activity occurred over a wider range of temperature, unlike the effect of cooling on the response in myelinated vagal fibres. It is concluded that the increase in urine flow caused by distension of a balloon in the left atrium is mediated solely by the Paintal-type atrial receptors which discharge into the myelinated fibres in the vagi.