Warm brain and eye temperatures in sharks.

Temperatures in the brain and eyes of mako and porbeagle sharks (Lamnidae) are 5 degrees C warmer than the water while the brain and eye temperatures in six other species of pelagic sharks are within 0.1 degrees C of water temperature. An orbital rete mirabile is present in the porbeagle and mako sharks but absent in the cranial vasculature of eleven other species of pelagic sharks. The orbital rete in the head of the porbeagle and mako sharks acts as a heat exchanger which conserves metabolic heat and raises the local tissue temperatures. This brain and eye warming system should buffer the central nervous system from the effects of rapid temperature change. Warming of the retina may improve the visual sensitivity of these active predators.

A brain heater in the swordfish.

The brain and eye of swordfish are warmer than the water. Associated with one of the eye muscles is a tissue that heats the brain. This brain heater is rich in mitochondria and cytochrome c and is supplied with blood through a vascular heat exchanger. It protects the central nervous system from rapid cooling during daily vertical excursions which may take the swordfish through a wide temperature range.

One why of the warmth of warm-bodied fish.

Tunas are unusual among fish in that they are warm bodied. In the present essay we argue that one adaptive advantage to being warm is that the warmth increases the rate of delivery of oxygen from the cell boundary to the mitochondria by myoglobin. This argument is supported by the following. 1) Tuna have extremely high rates of oxygen uptake, much higher than other fish and close to the rates achieved by mammals. 2) Tuna have an extraordinary capacity to maintain high cruising speeds for a long time. 3) Tuna have much red muscle that contains a high concentration of myoglobin. 4) The effect of temperature on simple diffusion of oxygen is very small whereas the potential affect on facilitated diffusion by myoglobin is large.

Effect of hydrostatic pressure on spectra of heme compounds.

Increase in hydrostatic pressure shifts the absorption bands of oxy-, carboxy-, and deoxyhemoglobin and myoglobin toward the red by 0.4 to 0.7 nm corresponding to a change in extinction coefficient of from 4 to 8% at the peak of the difference spectrum. The pressure difference spectrum for oxyhemoglobin closely resembles the difference spectrum described by Adams and Schuster ((1974) Biochem. Biophys. Res. Commun. 58, 528-533) following addition of inositol hexaphosphate to oxyhemoglobin. A similar shift was observed for derivatives of dimethyl-deuterohemedisulfonate in both Fe2+ and Fe3+ forms indicating that the protein is not required for this effect, in contrast to earlier reports of T. L. Fabry and J. W. Hunt ((1968) Arch. Biochem. Biophys. R3, 428-429) and Q.H. Gibson and F.G. Carey ((1975) Biochem. Biophys. Res. Commun. 67, 747-571) who were unable to observe changes in aqueous solutions of protoheme derivatives.

Effect of hydrostatic pressure on ligand binding to hemoglobin.

Increase in hydrostatic pressure to 1000 atm increased the affinity of human and menhaden (Brevoortia tyrannus) hemoglobins for oxygen. With necessary assumptions about the form of the equilibrium curve, and after correction for changes in pH and volume due to pressure, the increase in affinity is about 2-fold for both hemoglobins. At pH 6.5, Hill's n for menhaden hemoglobin is near 1, and it is believed to remain in the T state, whereas human hemoglobin undergoes a T to R transition. This suggests that the R-T equilibrium is not disturbed by pressure. In direct experiments the binding of a fluorescent effector (8 hydroxy-1,3,6-pyrene (trisulfonic acid) to deoxyhemoglobin was not changed by pressure. The binding of n-butylisocyanide to hemoglobin and to myoglobin is also greater at high pressures, similarly suggesting that the R-T transition is not involved in the pressure effect.