Effect of Temperature on the Heart and Ventilation Rates in the Agamid Lizard Uromastyx microlipes (the Dhubb) in the Central Region of Saudi Arabia

The effect of body warming and cooling on the heart and breathing rates has been studied in the agamid lizard Uromastyx microlipes. Electrocardiograph (ECG) of Uromastyx microlipes consists of P, QRS, and T components similar to those in mammals. Heart rate increases with body temperature. During warming the heart rate ranged from 7-134 beat/min, while during cooling ranged from 2-73 beat/min. The mean of heart rate during warming and cooling was not significantly different except at body temperatures of 20oC and 35oC. In two lizards heart rate was higher during warming than cooling for all temperatures. Both breathing types (thoracic and buccal) increased in frequency with body temperature. The range of thoracic breathing rates during both warming and cooling was 0-20 breath/min. Buccal breathing rate was 0-24 breath/min during warming, and 0-26 breath/ min during cooling. Introduction Animals can be classified into two types according to the stability of body temperature: hometherms and poikilotherms. Other classification based on the source of body heat; i.e. endothermic and ectothermic animals. Reptiles are considered poikilotherms animals but often able to maintain their body temperatures at a remarkably high and constant level throughout much of the day, by varying their exposure to the available sources of heat. They bask in the sun or rest on warm rocks when they get cold while if they get too hot they shelter under vegetation or in holes. Consequently, reptiles are sometimes termed ‘ectothermic or hometherms’ because of their ability to use external sources of heat to maintain body temperature. Mohammed S.A.D. Al-Ghamdi 22 In reptiles heart rate is related to different factors such as temperature, size, metabolism, respiratory state, and level of excitement[1]. Those various factors are connected together and it is difficult to separate their effects. In general, ambient temperature plays a major role in controlling metabolism in reptiles. Cooling and warming the animal directly affects the heart rate[1,2-6]. Presumably, the increase in heart rate with temperature supports an elevated cardiac output which augments O2 transport [1]. Reptiles can move between thermal exchange with the environment to maintain their body temperature, whereas birds and mammals control their temperature by shifts of their metabolic rates. In some lizards[2-5] and crocodilians[1] heart rate was higher during warming than cooling in the same temperatures. The heart rate is inversely related to temperature in most reptiles (increased heart rate as a result of body temperature increase)[7-10]. Three factors determined lung ventilation in most reptiles; the frequency of breathing, tidal volume and the duration of the pause period which interrupts the ventilatory[11]. In some lizards and snakes O2 uptake can increase twenty-fold above resting values with almost no increase in ventilation frequency[12-14], due to an increase in tidal volume. The effect of temperature on the ventilatory frequency has been studied in several species of reptiles[15-21]. The ventilation frequency increased as body temperature was raised. Consequently, the pause period was reduced[22]. In the lizard Pogona vitticeps the minute ventilation decreased at lower temperature as a result of a decrease in average frequency, and the tidal volume was temperature independent[23]. There is another factor behind the dependency of ventilation on changing of body temperature which is common to most reptiles; that is they maintain their blood pH in certain body temperature by controlling their ventilation, since blood pH shows a nearly linear inverse relationship with body temperature[23]. In Uromastyx microlipes, we attempted to determine the effect of body temperature on heart and ventilation rates and to compare it with other reptiles. Materials and Methods Seven lizards (Uromastyx microlipes) of either sex (350-1033 g) were used in this study. All lizards were collected from Riyadh. They were kept in large cage size 1.8 meters square × 1.2 meters high with its floor filled with fine coarse silver sand. In each cage there were two basking areas heated by 240 watt pig rearing lamps and two long tube light; one is a 40 watt (U.V.), and the other tube is 40 watt (U.V.B./U.V.A.). Both lights were run simultaneously to ensure the correct quantities of U.V./U.V.B./U.V.A. The power coming on at 8 am and off at 8 p.m., therefore, the lighting was on for 12 hours and the temperature in the Effect of Temperature... 23 cage was maintained in the range of 18-20oC during the night and 30-32oC during the day time and by mid-afternoon they can bask at between 45-47oC. Humidity was kept between 54%-65%. Food (vegetables and fruits) was provided daily with live adult locusts 2 or 3 times weekly. Details of the animal husbandry involved in the maintenance of wild caught Uromastyx has been reported[25]. Dhubbs were acclimated to this environment for at least 5 weeks before any experiment. Each animal was lightly anaesthetized with sodium pentobarbitone (Sagatal May & Baker Ltd) 20 mg kg–1 i.p. A rectal probe (Digitron instrumentation 3200K) was inserted through the cloacal opening to measure rectal temperature and a lamp was used to warm the animal. The ECG and EMG were recorded by inserting couple of bipolar wire electrodes of either copper (0.2-0.3 mm diameter) or stainless steel (0.006 inch diameter Johnson Matthey Metals Ltd.) subcutaneously; first couple were inserted in the chest close to the heart. The other was inserted to the end of thoracic cage near the lungs. This configuration invariably recorded the electrical activity of the heart and the intercostal muscles when active. The signals were fed into a preamplifier (Isleworth Electronics Type A101) and then further amplified and filtered (5-5000 Hz) (Neurolog NL 105, 106, 115, 120, 125), then led into an intelligent computer interface (1401 CED system) and displayed on a computer using data capture software (Spike2 CED). The data was sampled at 100-1000 Hz and stored on hard disk for subsequent analysis. Each lizard was first heated over a range of temperatures beginning at 15oC and increasing in the following steps 20oC, 25oC, 30oC, 35oC and 40oC. Then the cooling experiment was turn over for the range of: 40oC, 35oC, 30oC, 25oC, 20oC, 15oC, 10oC. The whole warming and cooling cycle took about 3 hours. 510 minutes was recorded at each temperature studied. Results The ECG of Uromastyx microlipes consists of P, QRS and T components (Fig. 1). The P wave is smaller relative amplitude and positive; Q is also smaller relative amplitude and negative; QRS complex is biphasic with a high relative amplitude positive R and low relative amplitude negative S wave, T wave in low temperature is similar to P wave, but in high temperature is higher than P wave (Fig. 1). The heart rate was recorded in seven lizards in both warming and cooling experiments, at the temperatures of 10oC, 15oC, 20oC, 25oC, 30oC, 35oC, and 40oC. The heart rate ranged from 7-134 beat/min during warming and 2-73 beat/min during cooling (Table 1). The results of the effect of both warming and cooling on the heart rate were illustrated in Table (1). Heart rate increases with inMohammed S.A.D. Al-Ghamdi 24 Temperatures L1-w L2-w L3-w L4-w L5-w L6-w L7-w Mean S.E. 15 12 18 16 8 7 11 8 11.43 1.60 20 22 23 23 29 11 28 12 21.14 2.69 25 32 55 24 58 19 50 20 36.86 6.44 30 41 72 33 44 27 36 31 40.57 5.68 35 69 73 69 82 72 58 72 70.71 2.69 40 85 134 80 116 101 76 102 99.14 7.89 TABLE 1. The warming and cooling experiments and their effect on heart rate (The mean and standard error were interacted). A. The warming experiments. B. The cooling experiments. creasing of body temperature (Fig. 2, 3). The heart rate was significantly higher during warming than cooling at two body temperatures; 20oC and 35oC (P = 0.01), whereas it was similar at all other temperatures. In two lizards (L2, W. 907 g and L3, W. 1033 g) heart rate was higher during warming than cooling at all temperatures (Table 4). FIG. 1. The normal ECG of Uromastyx microlipes, at body temperature 25oC ÆThe P, QRS and T components were indicated: 1. One cycle of ECG. 2. Two cycles of ECG.