alveolar minute ventilation, there was an optimum combination of ventilation .... Instantaneous breathing frequency (f). (min"1). Ventilatory dead space (VD).
J. exp. Biol. 113, 203-214 (1984) Printed in Great Britain © The Company of Biologists Limited 1984
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THE INTERRELATIONSHIP BETWEEN PULMONARY MECHANICS AND THE SPONTANEOUS BREATHING PATTERN IN THE TOKAY LIZARD, GEKKO GECKO BY WILLIAM K. MILSOM Department of Zoology, University of British Columbia, Vancouver, British Columbia, V6T2A9, Canada Accepted 9 April 1984
SUMMARY
The normal breathing pattern of the Tokay gecko (Gekko gecko) consists of single breaths or bursts of a few breaths separated by periods of breath holding. Increases in pulmonary ventilation that accompany rises in body temperature are caused by increases in respiratory frequency due to shortening of the periods of breath holding. Tidal volume and breath duration remain relatively constant. Measurements of the mechanical work associated with spontaneous breathing yielded values that were similar to those calculated for breaths of the same size and duration based on work curves generated during pump ventilation of anaesthetized animals. In this species, the pattern of periodic breathing and the ventilatory responses to changes in respiratory drive correspond with predictions of optimal breathing patterns based on calculations of the mechanical cost of ventilation. Bilateral vagotomy drastically alters the breathing pattern producing an elevation in tidal volume, a slowing of breathing frequency, and a prolongation of the breath duration. These alterations greatly increase the mechanical cost of ventilation. These data suggest that periodic breathing in this species may represent an adaptive strategy which is under vagal afferent control and which serves to minimize the cost of breathing.
INTRODUCTION
In a previous paper (Milsom & Vitalis, 1984), it was shown that the total work required to pump ventilate the Tokay gecko resulted primarily (>70%) from the work required to overcome elastic forces resisting lung inflation. For each level of alveolar minute ventilation, there was an optimum combination of ventilation frequency and tidal volume at which minute work was minimum. These data would suggest that for reptiles, as for mammals (Otis, Fenn & Rahn, 1950), there is a breathing pattern that optimizes the mechanical cost of ventilation. The purpose of the present study was to examine how the spontaneous breathing patterns of the Tokay gecko, under control conditions and following warming, compared with the predicted optimum breathing patterns based on the mechanical studies which were performed on anaesthetized animals using continuous pump ventilation. In addition, Key words: Pulmonary mechanics, control of breathing, reptiles.
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the effects of bilateral vagotomy on ventilation and the cost of breathing were also determined. METHODS
Experiments were performed on specimens of the Tokay gecko (Gekko gecko) (36-151 g) obtained commercially and maintained in glass aquaria at 22-23 °C. This species is primarily nocturnal and tropical in distribution being found commonly throughout the Philippine Archipelago. Although 22-23 °C is probably below the preferred temperature of this species, all individuals fed heartily on live locusts and grew rapidly during the 2-3 months before experiments were conducted. Food was withheld from each individual for approximately 1 week before it was used in any experiment. Pulmonary ventilation was measured using a head-body plethysmograph as described by Gratz (1978). The volume of the body chamber was 600 ml and the opening between the head and body chambers was surrounded by a plastic collar held in place by wing nuts. A metal ring was sutured to the neck of each lizard after complete infiltration of the neck with local anaesthetic and the head of each lizard was then passed through a small hole in a sheet of rubber dental dam. The metal ring was used to tether the animal to the opening between head and body chambers and the dental dam was secured by the plastic collar surrounding the opening, providing an air-tight seal between the two chambers. The hole in the dental dam was designed to be loose enough to allow swallowing movements, yet snug enough to be air-tight with the aid of stopcock grease or petroleum jelly. All animals were allowed to adjust to the chamber for 24 h before any measurements were made. The body chamber was open to air through a single opening fitted with a Fleisch no. 00 pneumotachograph. Air flow across the pneumotachograph was sensed with a Validyn DP 103-18 differential pressure transducer and this air flow signal was fed through a Gould integrating amplifier to give tidal volume (VT). Measurements of respiratory airflow and tidal volume associated with spontaneous ventilation were made on six animals at both 24 and 34 °C, and recorded on a Gould chart recorder. In three of the animals, a saline-filled catheter was also introduced through the skin into the body cavity at the level of the lungs and stitched in place. Since the lungs of this species lie in a pleuroperitoneal cavity, intrapulmonary and intra-abdominal cavity pressures must be in equilibrium. The movements of the ribs which power ventilation create changes in intra-abdominal cavity pressure which, when transferred to the lungs, generate air flow. Thus in spontaneously breathing animals, intraabdominal and intrapulmonary pressure are equivalent. In these animals, the measurements of intra-abdominal pressure, made with a Statham P23V pressure transducer and Gould transducer amplifier, as well as ventilatory air flow and tidal volume, were continuously recorded on the chart recorder during spontaneous ventilation and the pressure and volume signals were also plotted on an Esterline Angus XY plotter. In a further three animals, recordings were made of respiratory air flow and tidal volume both before and after bilateral vagotomy. Vagotomies were performed under
Breathing pattern regulation in lizards
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Halothane anaesthesia and animals were allowed to recover for at least 24 h before post-vagotomy measurements were made. Following all experimental runs, the animals were killed by anaesthetic overdose and measurements of resting lung volume ( VLR) and ventilatory dead space ( VD) were made. Resting lung volume was determined by removing air from the lungs with a syringe, via an intratracheal cannula, after opening the cannula to atmosphere with the animals placed in a normal supine position. The lungs and trachea, complete with glottis were then removed from each animal and the trachea tied closed at the lung hilus. The dead space volume was then determined by carefully measuring the volume of water required to fill the trachea from lung hilus to glottis. All measurements in this paper are given as means ± s.E. Direct comparisons of variables in all Tables were tested with the Student f-test.
RESULTS
The ventilatory pattern associated with normal resting conditions in the Tokay gecko is depicted in Fig. 1. The effects of warming to 34 °C and of spontaneous activity are shown for comparison. Mean values of respiratory variables measured during resting breathing at 24 and 34 °C are given in Table 1 along with values obtained from anatomical measurements. At 34 °C, tidal volume ( V T ) is relatively unchanged but breathing frequency (f) is elevated solely by reducing the periods of breath holding ( T N V P ) . The duration of each individual breath (T to t) remains unchanged. The number of breaths taken during each breathing episode (breaths/ ventilatory period, VP) remains the same but the period between episodes is reduced until breathing becomes almost continuous. Since Ttot does not change, the instantaneous breathing frequency (f = 60/T t o t) does not change appreciably. Both
Table 1. Values of respiratory variables and parameters during resting ventilation in the Tokay gecko at various temperatures 24°C Tidal volume (VT) Ventilation frequency (f) Minute ventilation (VE) Breaths/ventilatory period Bursts/min Alveolar minute ventilation (VA) Breath duration (T,,,) Non-ventilatory period (TNVP) Instantaneous breathing frequency (f) Ventilatory dead space (VD) Resting lung volume VT VD
-
xl00
1
(ml 100 g' ) (miiT 1 ) (mlmin-'lOOg" 1 )
34 °C
l-0± 34 ± 28 ± 2-1 ± 20-8 ± 17-0» 1-5 ± 2 ±
l-2±0-l 9 ±2 10 ±3 1-7 ±0-3 6 0 ±1-6
(mlmin-'lOOg- 1 ) (s) (s) (min" 1 ) (mllOOg"1) (ml 100 g"')
VLK + V T
^ '
Weight
(g)
6-3 1-6 ±0-2
14 ± 6 37 ±7
40 0-50 ± 0-03 6-3 ± 1-0
0-2 9* T 1-4 5-6*
0-2 0-1* ± 10
7-9
100
±21
• Indicates values recorded at 34°C were significantly different (/>
