ABSTRACT
Ventilation volume Vg - mlH2O.min-1 ), respiratory frequency (fR - breaths.min-1) and tidal volume (VT - mlH2O.breath-1 ) were measured in a group of Piaractus mesopotamicus (650.4 +/- 204.7 g; n = 10) during normoxia and in response to graded hypoxia. The fR was maintained constant, around 100 breaths.min-1, from normoxia until the O2 tension of the inspired water (PiO2) of 53 mmHg, below which it increased progressively, reaching maximum values (157.6 +/- 6.3 breaths.min-1) at 10 mmHg. The VT rose from 1.8 +/- 0.1 to 6.0 +/- 0.5 and 5.7 +/- 0.4 mlH2O. breath-1 in the PiO2 of 16 and 10 mmHg, respectively. The Vg increased from 169.3 11.0 (normoxia) to 940.1 +/- 85.6 mlH2O. min-1 at the PiO2 of 16 mmHg, below which it also tended to decrease. A second group of fish (29 to 1510.0 g, n = 34) was used for the evaluation of allometric relationships concerning ventilation and dimensions of the buccal and opercular cavities. At maximum Vg, the VT corresponded to 93.2 +/- 2.4% of the buccal volume and 94.9 2.3% of the opercular volume, suggesting that the Vg of P. mesopotamicus is limited by the volumes of buccal and opercular cavities in severe hypoxia.
ABSTRACT
The cardiac responses of Oreochromis niloticus acclimated to 25 degrees C were assessed using ventricle strips mounted for isometric force recording (Fc) and in vivo heart rate (f(H)). f(H) increased progressively from 25 to 40 degrees C. At extracellular Ca(2+) concentrations of 1.25 and 9.25 mM, a transition from 25 to 40 degrees C resulted in a decreased Fc. At both 25 and 40 degrees C, Fc rose when [Ca(2+)] was increased from 1.25 to 9.25 mM. Fc remained constant at 72 and 120 contractions.min(-1) at 25 and 40 degrees C, respectively, and declined thereafter. The post-rest potentiation was not influenced by ryanodine, indicating that the sarcoplasmic reticulum is not important to the excitation-contraction coupling.
