O2-Filled Swimbladder Employs Monocarboxylate Transporters for the Generation of O2 by Lactate-Induced Root Effect Hemoglobin

Takahiro Umezawa,Michael F Romero,Zinia Islam,Maho Ogoshi,Yoko Yamamoto,Keijiro Munakata,Shigehisa Hirose,Hiroyuki Doi,Kayoko Ookata,Akira Kato,Lewis George Halsey

doi:10.1371/journal.pone.0034579

Abstract

The swimbladder volume is regulated by O2 transfer between the luminal space and the blood In the swimbladder, lactic acid generation by anaerobic glycolysis in the gas gland epithelial cells and its recycling through the rete mirabile bundles of countercurrent capillaries are essential for local blood acidification and oxygen liberation from hemoglobin by the “Root effect.” While O2 generation is critical for fish flotation, the molecular mechanism of the secretion and recycling of lactic acid in this critical process is not clear. To clarify molecules that are involved in the blood acidification and visualize the route of lactic acid movement, we analyzed the expression of 17 members of the H+/monocarboxylate transporter (MCT) family in the fugu genome and found that only MCT1b and MCT4b are highly expressed in the fugu swimbladder. Electrophysiological analyses demonstrated that MCT1b is a high-affinity lactate transporter whereas MCT4b is a low-affinity/high-conductance lactate transporter. Immunohistochemistry demonstrated that (i) MCT4b expresses in gas gland cells together with the glycolytic enzyme GAPDH at high level and mediate lactic acid secretion by gas gland cells, and (ii) MCT1b expresses in arterial, but not venous, capillary endothelial cells in rete mirabile and mediates recycling of lactic acid in the rete mirabile by solute-specific transcellular transport. These results clarified the mechanism of the blood acidification in the swimbladder by spatially organized two lactic acid transporters MCT4b and MCT1b.

Highlights

The swimbladder is a gas-filled internal organ that controls the body buoyancy of teleost fish, help them to stay at a chosen water depth without wasting energy
The gas gland cells were adjacent to capillary endothelial cells (Figs. 1C, 2A) which provided a connection to the rete mirabile bundle of capillaries (Fig. 1B, D)
As expected from previous studies that demonstrated the production of lactic acid in gas gland cells, we identified a monocarboxylate transporter, MCT4b, in the gas gland cells

Summary

Introduction

The swimbladder is a gas-filled internal organ that controls the body buoyancy of teleost fish, help them to stay at a chosen water depth without wasting energy. The O2 partial pressures in the swimbladders of living fish are much higher than those in the circulating blood and the surrounding water [2], O2 can be transported against the gradient as a result of the Root effect [4,5,6]. This effect involves a markedly reduced capacity of fish hemoglobin to bind O2 at low pH [7,8]. Hemoglobin molecules can act as acid-controlled molecular oxygen pumps that deliver O2 against a high oxygen concentration gradient to the swimbladder. Local blood acidification in the swimbladder is essential for luminal O2 secretion

Methods

Results

Conclusion