Does digoxin sensitize left ventricular mechanoreceptors?

Afferent nerve fibre activity from left ventricular mechanoreceptors was recorded in 10 anaesthetized cats before and after two intravenous injections of 15 micrograms/kg digoxin at 1 hour interval. These receptors are activated by coronary artery occlusion and induce a depressor cardiovascular reflex resulting in bradycardia and hypotension. Neither the spontaneous activity of the receptor's afferent nerve fibres nor their maximum activity during temporary coronary artery occlusion was affected by digoxin. The results show that digoxin in therapeutic doses has no sensitizing effect on left ventricular mechanoreceptors with vagal afferent fibres. The sensitization of cardiopulmonary baroreflexes by digitalis glycosides shown in previous investigations is thus more likely to be mediated by a central nervous effect of the drug.

Method for measurement of heart rate in awake, noninstrumented small mammals.

A method is described that enables the researcher to determine the heart rate in awake, noninstrumented small animals by recording the electrocardiogram (ECG) via the paws. Single animals are placed in a cage that has metal plates built into its floor. Through switches any two plates can be connected with an ECG recorder whenever contact with at least one front and one hind paw is made. The heart rate is then determined by measuring the number of R waves per unit of time. In rats of 140 and 300 g body wt mean resting heart rates of 384 +/- 10 and 320 +/- 4 beats/min, respectively, have been measured with this method.

Endoanaesthesia of left ventricular mechanoreceptors by steady state infusion of lignocaine and the influence of dopamine.

We recorded the afferent activity of 11 left ventricular mechanoreceptors in filaments of the vagus nerve in 10 chloralose-anaesthetised cats. The fibres showed low irregular spontaneous activity of 2.9 (0.2 to 8.4) spikes X s-1. During temporary occlusion of the left anterior descending or left main coronary artery they were activated to 19.1 (4.8 to 47.0) spikes X s-1. Intravenous infusion of 0.175 and 0.35 mg X kg-1 X min-1 lignocaine lowered heart rate and blood pressure. The spontaneous nerve fibre activity remained unchanged by the local anaesthetic, whereas the maximum activity evoked by coronary artery occlusion was reduced to 14.7 (2.6 to 34.1) and 10.9 (2.4 to 27.6) spikes X s-1. An additional infusion of 5 and 10 micrograms X kg-1 X min-1 dopamine during continued application of 0.35 mg X kg-1 X min lignocaine raised heart rate and blood pressure to control values but had only minimal effects on the response of the fibres to coronary occlusion. It is concluded that lignocaine exerts a specific endoanaesthetic effect on the left ventricular mechanoreceptors, which is not mediated by its negative inotropic side effect.

Capillary density in rat myocardium during timed plasma staining.

In urethan-anesthetized thoracotomized rats the density of myocardial capillaries that had been stained during timed infusions of a plasma label was determined. gamma-Globulin-coupled fluorochromes [lissamine-rhodamine B 200 (RB 200) and fluoresceinisothiocyanate (FITC)] were applied to the vascular system for different intervals of time. The circulation in the heart was freeze stopped, and the dye-containing capillaries were counted in histological cross sections of the organ. When the dyes had been infused into the left atrium for 1, 2, 5, or 10 s the number of stained vessels continuously increased (1,960 +/- 170, 2,780 +- 160, 3,310 +/- 130, and 3,340 +/- 140 cap/mm2 in the subepicardium and 1,990 +/- 210, 2,490 +/- 190, 3,090 +/- 160, and 3,210 +/- 160 cap/mm2 in the subendocardium, respectively). However, further prolongation of dye exposure (2, 10, or 30 min) did not increase significantly the number of marked capillaries. The induction of a general hypoxia (30-s stop of ventilation) after dye application also did not induce higher capillary counts than were found in the normoxic animals during long-time plasma labeling. Changes in red blood cell-containing capillaries were not covered by these observations. The distribution of stained capillaries in the evaluated areas was more inhomogeneous after incomplete filling with RB 200 for 1 s than after complete staining with FITC for 4 min. The results favor the view that in the rat heart at rest the blood plasma passes the microcirculation in an inhomogeneous manner; in all capillaries the slowest transit times do not exceed 5 s, however.

Excitation of afferent fibres in the cardiac sympathetic nerves induced by coronary occlusion and injection of bradykinin. The influence of acetylsalicylic acid and dipyron.

Afferent impulse activity was recorded in single fibres of the inferior cardiac sympathetic nerve of the cat. When the descending branch of the left coronary artery was ligated for 60 sec an enhancement of afferent impulses was recorded. Elevations in discharge frequency were also induced by injecting bradykinin, epinephrine, and isoprenaline or by general hypoxia due to interruption of the artificial ventilation. When these procedures were after pretreatment with the analgesic agents, acetylsalicylic acid or dipyron a reduction in spike discharge was observed only with bradykinin after application of acetylsalicylic acid. No influence of these pretreatments on the effects of coronary occlusion, general hypoxia and injection of epinephrine and isoprenaline could be observed. These results suggest that bradykinin does not predominate as mediator substance in eliciting ischemic heart pain.