Thermal sensitivity of mitochondrial respiration efficiency and protein phosphorylation in the clam Mercenaria mercenaria.

The mitochondria of intertidal invertebrates continue to function when organisms are exposed to rapid substantial shifts in temperature. To test if mitochondrial physiology of the clam Mercenaria mercenaria is compromised under elevated temperatures, we measured mitochondrial respiration efficiency at 15 degrees C, 18 degrees C, and 21 degrees C using a novel, high-throughput, microplate respirometry methodology developed for this study. Though phosphorylating (state 3) and resting (state 4) respiration rates were unaffected over this temperature range, respiratory control ratios (RCRs: ratio of state 3 to state 4 respiration rates) decreased significantly above 18 degrees C (p < 0.05). The drop in RCR was not associated with reduction of phosphorylation efficiency, suggesting that, while aerobic scope of mitochondrial respiration is limited at elevated temperatures, mitochondria continue to efficiently produce adenosine triphosphate. We further investigated the response of clam mitochondria to elevated temperatures by monitoring phosphorylation of mitochondrial protein. Three proteins clearly demonstrated significant time- and temperature-specific phosphorylation patterns. The protein-specific patterns of phosphorylation may suggest that a suite of protein kinases and phosphatases regulate mitochondrial physiology in response to temperature. Thus, while aerobic scope of clam mitochondrial respiration is reduced at moderate temperatures, specific protein phosphorylation responses reflect large shifts in function that are initiated within the organelle at higher temperatures.

The histological localization of heparin in the northern quahog clam, Mercenaria mercenaria.

The cellular location of heparin in Mercenaria mercenaria, the northern quahog clam, was identified histologically by azure II and alcian blue/safranin staining. A survey of various organs indicates that heparin was found in several morphological cell subpopulations of the labial palp, ctenidia, intestine, siphon, and pallium. Cells containing these compounds were restricted to areas proximal to epithelial surfaces and granules were observed in all of the cell types except those of the intestine. The "mast-like" cells that we report here may play a role analogous to that of mammalian mast cells in inflammatory responses, using heparin proteoglycan as a supportive defense molecule.