Low guanylyl cyclase activity in Weddell seals: implications for peripheral vasoconstriction and perfusion of the brain during diving

Abstract

Nitric oxide (NO) is a potent vasodilator, which improves perfusion and oxygen delivery during tissue hypoxia in terrestrial animals. The vertebrate dive response involves vasoconstriction in select tissues, which persists despite profound hypoxia. Using tissues collected from Weddell seals at necropsy, we investigated whether vasoconstriction is aided by downregulation of local hypoxia signaling mechanisms. We focused on NO–soluble guanylyl cyclase (GC)-cGMP signaling, a well-known vasodilatory transduction pathway. Seals have a lower GC protein abundance, activity, and capacity to respond to NO stimulation than do terrestrial mammals. In seal lung homogenates, GC produced less cGMP (20.1 ± 3.7 pmol·mg protein−1·min−1) than the lungs of dogs (−80 ± 144 pmol·mg protein−1·min−1 less than seals), sheep (−472 ± 96), rats (−664 ± 104) or mice (−1,160 ± 104, P < 0.0001). Amino acid sequences of the GC enzyme α-subunits differed between seals and terrestrial mammals, potentially affecting their structure and function. Vasoconstriction in diving Weddell seals is not consistent across tissues; perfusion is maintained in the brain and heart but decreased in other organs such as the kidney. A NO donor increased median GC activity 49.5-fold in the seal brain but only 27.4-fold in the kidney, consistent with the priority of cerebral perfusion during diving. Nos3 expression was high in the seal brain, which could improve NO production and vasodilatory potential. Conversely, Pde5a expression was high in the seal renal artery, which may increase cGMP breakdown and vasoconstriction in the kidney. Taken together, the results of this study suggest that alterations in the NO-cGMP pathway facilitate the diving response.

microRNA profiling in the Weddell Seal suggests novel regulatory mechanisms contributing to diving adaptation

“…Within the brain, an additional upregulation of novel-25-5p and novel-119-5p was detected in adult seals, which also target Gucy1b1 (Table S8). Guanylyl cyclase is a major target of NO and has previously been shown to be reduced in seal tissues [10]. The alpha-1 subunit (Gucy1a1) is also targeted by several DE miRNAs, including novel-128-5p ( Fig.…”

“…In diving seals, a NO response could be detrimental, as some peripheral vascular resistance is required to maintain central arterial pressure with submergence bradycardia. Indeed, downregulation of the nitric oxide-guanylyl cyclase-cGMP signal transduction pathway is limited in Weddell seals [10], which may facilitate central nervous system control of the dive response, but also reduce production of harmful peroxynitrites The number of DE miRNAs targeting this pathway, in particular the number of novel miRNAs with this predicted function strongly suggest that lineagespecific miRNA regulation contributes to control of local vasoregulators in the Weddell seal, and perhaps the evolution of diving capabilities. The most significant miRNA target of this pathway in the dataset is NO synthase 1 (Nos1).…”

“…Samples were collected from Weddell seal adults and pups near McMurdo Station, Antarctica as part of a larger study [10]. Tissue samples (heart right ventricle, longissimus dorsi swimming muscle, cerebral cortex) were collected at necropsy from animals found dead of natural causes on the sea ice (pups were generally ~1 week of age).…”

“…Tissues were rinsed in cold PBS, blotted dry, then either snap frozen in liquid nitrogen or preserved in an RNA stabilization solution (RNAlater, ThermoScientific AM7021). Venous blood samples were collected into EDTA-vacutainers from healthy adults and weaned pups during a brief handling period as previously described [10]. Plasma was isolated from whole blood by centrifugation (3000g for 15min) within an hour of collection in Antarctica.…”

“…respiratory pigments), tissue iron content and metabolic enzyme levels [5]. However, the details of the extent of tissue-specific maturation to refine local blood flow [10], metabolic control, and to combat negative effects of hypoxia exposure are still to be elucidated. Interestingly, pup physiology develops during weaning and throughout a post-weaning fast, including cardiac ontogeny to develop the fine-scale control of bradycardia observed in adults.…”