The nature of the atrial receptors responsible for a reflex increase in heart rate in the dog.

Abstract

SUMMARY1. In dogs anaesthetized with chloralose, small latex balloons were positioned at the left pulmonary vein-atrial junctions so as to stretch this region. By recording action potentials from slips of the cervical vagi it was established that distension of these balloons stimulated receptor endings in the atrial endocardium which discharged into the myelinated branches of the vagi i.e. Paintal type A, type B and Intermediate type receptors.2. In other dogs, cooling the cervical vagus in steps of 2 'C reduced this response in vagal myelinated fibres. With twelve receptors the response to distension was reduced by 30 % when the vagus was cooled to 16 'C , by 70 % when cooled to 12 'C and abolished at 8-12 'C.3. In a third group of dogs, distension of balloons at the pulmonary vein-atrial junctions resulted in a reflex increase in heart rate. Cooling the cervical vagi in these dogs in stages to 8 'C reduced this increase in heart rate. In nine dogs the response was reduced by 20 % when the vagi was cooled to 16 'C, by 70 % when cooled to 12 'C and abolished between 12 and 8 TC.4. In a fourth group of dogs, distension of balloons at the pulmonary vein-atrial junctions was shown also to activate receptor endings in the atria which discharged into non-myelinated branches of the Vagi. In twelve receptors, cooling the cervical vagus in steps of 2 'C reduced this evoked increase in activity in non-myelinated fibres. These responses were abolished over a wide range of temperature unlike the responses observed above.5. It is concluded that the increase in heart rate which follows distension of balloons at the pulmonary vein-atrial junctions is mediated solely by the Paintaltype receptors which discharge into the myelinated fibres in the vagi.

Plasma vasopressin concentration in the anaesthetized dog before, during and after atrial distension.

“…At a temperature of 8-12 'C increases in the frequency of discharge, associated with atrial distension, in myelinated and some non-myelinated afferent fibres are blocked (Kappagoda, Linden & Sivanathan, 1979); and some vagal efferent fibres may also be blocked. It was therefore of interest to discover that vagal cooling by itself led to a significant rise of plasma AVP concentration (Table 1).…”

The Responses in Renal Nerves to Stimulation of Atrial Receptors, Carotid Sinus Baroreceptors and Carotid Chemoreceptors

“…Experiments were performed on twenty dogs weighing 18-28 kg which were given a subcutaneous injection of morphine sulphate (0 5 mg. kg-') and subsequently anaesthetized with chloralose and artificially ventilated with oxygen-enriched air (40% 02) as described by Kappagoda et al (1979). During recording of the activity in renal nerves, the animals were given gallamine triethiodide (initial dose 2 mg. kg-'; subsequent doses 1 mg. kg-') as described by Linden et al (1980); periodically doses of gallamine triethiodide were omitted and the animal allowed to recover from its effects so as to gauge the depth of anaesthesia.…”

“…Karim, Kidd, Malpus & Penna (1972) provided direct evidence for an increase in activity in efferent cardiac sympathetic nerves in response to stimulation of atrial receptors and a reflex reduction in activity in efferent renal sympathetic nerves. Subsequently both the reflex increase in heart rate and the reflex reduction in activity in renal nerves were shown to involve only atrial receptors attached to myelinated vagal fibres (Kappagoda, Linden & Sivananthan, 1979;Linden, Mary & Weatherill, 1980). Carswell, Hainsworth & Ledsome (1970) observed that the magnitude of the reflex increase in heart rate was unaltered by changes in pressure in the carotid sinuses implying little or no influence of changes in discharge of carotid sinus baroreceptors on this response.…”

Effects of combined carotid chemoreceptor and atrial receptor stimulation on renal blood flow in anaesthetized dogs.

“…The vagi were cooled in the neck using a silver thermode, the details of which have been described previously (Kappagoda, Linden & Sivananthan, 1979).…”

“…In all experiments the cervical vagi were cooled by a thermoelectric module (see Kappagoda et al 1979 for details). In seven dogs the vagi were cooled to a temperature of 6-8 'C and in one dog to a temperature of 3 TC.…”