Metabolism-weight relationship in 17 humming-bird species at different temperatures during day and night

Abstract

Experientia 37 (1981), Birkh/iuser Verlag, Basel (Schweiz) 1307 rate. The mean rate of discharge of the isolated AVN was 72_+ 11/rain. 10 CT preparations, when isolated from the musculi pectinati, continued discharging. These were subsequently cut transversely into 2-4 pieces: after variable times (up to 1 min) each discharged at a rate varying between the initial rate of the entire CT and its half value. In 3 of the preparations, 1 CT piece did not show automaticity. Action potentials with a Variable slope of diastolic depolarization were recorded from a limited area of the deep layer of any spontaneously discharging isolated CT-musculi pectinati preparation, as shown in figure ia. The amplitude of these action potentials was between 80 and 90 mV and their Vma x between 90 and 140 V/see. The location of the pacemaker area varied within the CT deep layer. Similar pacemaker potentials were recorded by Paes de Carvalho et alp from the sinoatrial ring bundle, but not from the superficial CT layer. An example typical of action potentials recorded from the deep layer outside the pacemaker region is shown in figure lb. These action potentials were homogeneous, had a mean amplitude of 114_+2.1 mV and a mean gmax of 374_ 14 V/sec. Vma x of action potentials were plotted against the take-off potentials of all records of the CT deep layer and a sigmoidal relationship was found ( fig.2). Experimental data are in good agreement with the sigmoidal curve drawn from the equation of Hodgkin and Huxley 9where, according to Weidmann l~ h is the fraction of the highest value observed for the rate of rise, V the take-off potential in mV, V h the potential at which h is half maximum and s the slope factor. Vh= 75.3 mV and s = 5.78 were determined according to Noma and Irisawa I1. The sigmoidal relationship found is similar to that reported by Weidmann 1~ in the ventricular Purkinje fibers but largely differs from that in sinoatrial pacemaker fiberslL Another resemblance to Purkinje fibers is the relatively steep rate of rise.The fine morphology and particularly the ultrastructural features of the mitochondria shown in figure 3, demonstrate the excellent in depth preservation of the CT, comparable to the state of preservation of the CT excised from hearts perfused by the Langendorff method 1. The present findings demonstrate that a) the CT deep layer fibers can act as pacemaker, discharging at a rate intermediate to the rates of the 2 nodes, b) Pacemaker activity, as a property of the deep layer cells, can be observed throughout the CT (all the pieces cut from it discharged spontaneously), c) The high discharge rate recorded from the isolated crista indicates that this area, being inherently faster than the AVN pacemaker, would probably be the faster pacemaker in the absence of SAN pacing, d) From the electrophysiological characteristics a resemblance of the CT deep layer pacemaker potentials and those of Purkinje fibers emerges.Summary. The mean metabolic rate during day and night of 17 different humming-bird species is considerably ...

Are Hummingbirds Facultatively Ammonotelic? Nitrogen Excretion and Requirements as a Function of Body Size

“…Indeed, the proportion of assayed nitrogen excreted as ammonia by 3.4-g broad-tailed hummingbirds (S. platycercus) feeding on a nitrogen-free diet ( , 31.13% ‫ע‬ 1.66% n p ) was very similar to that excreted by A. alexandri 16 ( ), and again there was no positive correlation 25.66% ‫ע‬ 2.71% with water intake (T. J. McWhorter and C. Martínez del Rio, unpublished data). Basal metabolic rates of small hummingbirds such as A. alexandri are 10%-15% higher than predicted based on body mass (Lasiewski 1963;Prinzinger et al 1981), whereas this is not the case for larger hummingbirds such as E. fulgens and L. clemenciae (Lasiewski and Lasiewski 1967). Higher metabolic demands necessitate higher energy intake and, as observed in A. alexandri, higher mass-specific rates of food and sucrose intake.…”

Extreme and variable torpor among high-elevation Andean hummingbird species

“…in the Andes Mountains, providing one of the most spectacular examples of avian adaptation to extreme environments. The challenges of living in these cold, wet and hypoxic environments are compounded by hummingbirds being among the smallest of endotherms and possessing the highest mass-specific metabolic rates of any vertebrates [1–3]. Pronounced thermoregulatory costs are combined with very high costs of hovering flight at high elevations [4,5] and a diet of flower nectar requiring daily intake rates sometimes exceeding hummingbirds' own body masses ( M b ) [6,7].…”