Antioxidant systems and anoxia tolerance in a freshwater turtle Trachemys scripta elegans

In the search for MBP phosphorylating activities in Dictyostelium discoideum, we have found a proteolysis-activated protein kinase. This activity which is distributed between the soluble and the particulate fractions of the cell, uses MBP and histone as substrate and has a molecular mass of 140 kDa as detected in an in situ' assay.This protein kinase has several features shared by the protein kinase C family, such as substrate specificity and sensitivity to proteolysis, but its molecular mass is much larger than that described for the known protein kinase C isoforms.To better characterize this activity we have studied its sensitivity to several protein kinase C inhibitors and activators. This protein kinase is activated neither by phorbol ester nor by phosphatidylserine or Ca²⁺. The activity is inhibited by staurosporine and PKC pseudosubstrate, but is not affected by the specific protein kinase C inhibitor bisindolylmaleimide.These data lead us to propose that proteolytically activated Dictyostelium protein kinase belongs to the recently described protein kinase C-related family.     

Specialization for underwater hearing by the tympanic middle ear of the turtle, Trachemys scripta elegans

Turtles, like other amphibious animals, face a trade-off between terrestrial and aquatic hearing. We used laser vibrometry and auditory brainstem responses to measure their sensitivity to vibration stimuli and to airborne versus underwater sound. Turtles are most sensitive to sound underwater, and their sensitivity depends on the large middle ear, which has a compliant tympanic disc attached to the columella. Behind the disc, the middle ear is a large air-filled cavity with a volume of approximately 0.5 ml and a resonance frequency of approximately 500 Hz underwater. Laser vibrometry measurements underwater showed peak vibrations at 500-600 Hz with a maximum of 300 μm s(-1) Pa(-1), approximately 100 times more than the surrounding water. In air, the auditory brainstem response audiogram showed a best sensitivity to sound of 300-500 Hz. Audiograms before and after removing the skin covering reveal that the cartilaginous tympanic disc shows unchanged sensitivity, indicating that the tympanic disc, and not the overlying skin, is the key sound receiver. If air and water thresholds are compared in terms of sound intensity, thresholds in water are approximately 20-30 dB lower than in air. Therefore, this tympanic ear is specialized for underwater hearing, most probably because sound-induced pulsations of the air in the middle ear cavity drive the tympanic disc.