Relative distribution of types A and B atrial receptors in dogs, cats, monkeys and rabbits

Type A atrial receptors in the cat: effects of changes in atrial volume and contractility

1. Action potentials were recorded from filaments of the right cervical vagus in anaesthetized, paralysed cats. Right atrial receptors with type A (twelve units) and Intermediate type (two units) patterns of spontaneous discharge were selected and their responses to changes in atrial volume were analysed. 2. Changes in atrial volume of similar magnitude were produced under four different conditions: a, innervated hearts; b, denervated hearts; c, depression of atrial muscle contractility induced after cardiac denervation and d, non-beating hearts. 3. In innervated hearts the systolic discharge of each receptor showed a characteristic response to changes in atrial volume. Cardiac denervation and depression of atrial contractility markedly altered this response in terms of frequency of discharge threshold and 'sensitivity'. 4. During increments in atrial volume all the receptors but one assumed an Intermediate pattern of discharge. The diastolic firing rate was, however, higher for any given atrial pressure, in innervated hearts than under conditions b, c and d. 5. In innervated hearts the response of the receptors to atrial systole was characterized by a higher frequency of discharge and a lower threshold with respect to the responses of the same receptors to atrial filling. These differences were minimized at high atrial volumes and during depression of atrial contractility. 6. The results indicate that the responses of the receptors to atrial systole are mainly dependent upon the state of contraction of atrial muscle and that the differences between systolic and diastolic discharge are mainly due to the high dynamic component of the stretch during atrial contraction.

Atrial receptors in the dog and rabbit

1. Action potentials were recorded from slips of the cervical vagi in anaesthetized dogs and rabbits. Single functional units with atrial patterns of discharge (Paintal Type A, B and intermediate) were obtained and then attempts were made to alter (i.e. convert) their patterns of discharge. Finally the points of origin of these action potentials were located.2. Thirty unselected units were investigated in thirty dogs. Twenty-seven of these were located in the endocardium of the vein-atrial system and the ratio of the type A, type B and intermediate type receptors was 1:16:10; three units were located elsewhere in the chest. Conversion of the pattern of discharge was achieved in twenty of the twenty-seven units; conversion was achieved in the single type A unit.3. In a second series of experiments in dogs, eight Paintal Type A units were selectively studied in fifteen animals. Four of these were located in the endocardium and all were converted. The remaining four were located outside the endocardium and conversion could not be achieved in two of these. Thus in the entire investigation, the ;type A' units which could not be converted were all located at sites other than the atrial endocardium.4. In the corresponding unselected study in the rabbit, eleven units were studied in eleven animals. Nine of these units were located in the atrial endocardium and the ratio of the type A, type B and intermediate type receptors was 2:1:6. Conversion was achieved in both type A units, the sole type B unit and two of the intermediate units. One of the two units found outside the atrial endocardium was a ;type A' unit and could not be converted.5. The present investigation has shown that the atrial receptors with a Paintal Type A pattern of discharge are relatively rare in both dogs and rabbits. Conversion of the pattern of discharge is a relatively common phenomenon. Evidence for the proposition that there is one basic type of atrial receptor whose pattern of discharge is determined by its precise location in the vein-atrial system is discussed.

Cardiopulmonary vagal afferents in the monkey: a survey of receptor activity

A survey was made of vagal afferents whose endings originated in cardiopulmonary areas of the Rhesus monkey. Recordings of action potentials from single fiber preparations of the left cervical vagus were made in both open and closed chest monkeys. A total of 425 receptors were identified in sixteen animals. These consisted of 347 pulmonary stretch receptors (one of which increased its discharge during expiration), 42 aortic baroreceptors, 4 ventricular pressure receptors, 1 epicardial ventricular receptor, 7 type A atrial receptors and 24 type B atrial receptors. The response of each cardiovascular receptor was tested by altering the stimulus for their discharge. Aortic and ventricular baroreceptors increased their discharge in response to an increase in blood pressure induced by intravenous norepinephrine. Type A atrial receptors did not increase their discharge in response to an increase in atrial pressure during intravenous administration of isotonic saline, while type B atrial receptors did. The discharge of the latter became continuous following the intravenous administration of veratridine sulphate (20 microgram). It is concluded that the basic types of cardiopulmonary afferents exist in the non-human primate and that they respond similarly to those which have been demonstrated in lower species.

Atrial receptors in the cat

Action potentials were recorded from slips of the cervical vagi in anaesthetized cats. Single functional units with atrial patterns of discharge (type A, B and Intermediate) were first obtained and then attempts were made to alter (i.e. convert) their patterns of discharge. Finally the points of origin of their action potentials were located. The investigations were done in five stages. 2. In the first series, thirty unselected units were investigated in thirty cats. Twenty-five of these were located in the endocardium of vein-atrial system and consisted of two type A, fifteen type B and eight Intermediate type units. The remaining five units were located elsewhere in the chest Conversion of the pattern of discharge was achieved in sixteen of the twenty-five atrial units. Both atrial type A units were converted. 3. In the second series, eight type A units were selectively studied in twelve cats. Five were located in the atrial endocardium and all were converted. Of the other three units which were located at other sites in the chest, one could not be converted. 4. In the third series, four type A units which could not be converted were selectively studied in twenty cats. All were located outside the atria. 5. In the fourth series, three type B units which could not be converted were selectively studied in six cats. These units were located in the pulmonary veins and in the lateral walls of the atria. 6. In the fifth series, fifty-five units were investigated in three anaesthetized spontaneously breathing cats. The proportion of the types of units were similar to that obtained in the artificially respired cats (first series). 7. The present study has shown that atrial receptors with a type A pattern of discharge are relatively rare in the cat and that conversion of the patterns of discharge is a common phenomenon. Evidence is presented which suggests that there is one basic type of atrial receptor whose pattern of discharge is determined by their precise location in the vein-atrial system.