Casper K. Larsen

Change of cardiac function, but not form, in postprandial pythons.

Pythons are renowned for a rapid and pronounced postprandial growth of the heart that coincides with a several-fold elevation of cardiac output that lasts for several days. Here we investigate whether ventricular morphology is affected by digestive state in two species of pythons (Python regius and Python molurus) and we determine the cardiac right-to-left shunt during the postprandial period in P. regius. Both species experienced several-fold increases in metabolism and mass of the digestive organs by 24 and 48 h after ingestion of meals equivalent to 25% of body mass. Surprisingly there were no changes in ventricular mass or dimensions as we used a meal size and husbandry conditions similar to studies finding rapid and significant growth. Based on these data and literature we therefore suggest that postprandial cardiac growth should be regarded as a facultative rather than obligatory component of the renowned postprandial response. The cardiac right-to-left shunt, calculated on the basis of oxygen concentrations in the left and right atria and the dorsal aorta, was negligible in fasting P. regius, but increased to 10-15% during digestion. Such shunt levels are very low compared to other reptiles and does not support a recent proposal that shunts may facilitate digestion in reptiles.

Structural and functional characterization of human and murine C5a anaphylatoxins.

The structure of the human C5aR antagonist, C5a-A8, reveals a three-helix bundle conformation similar to that observed for human C5a-desArg, whereas murine C5a and C5a-desArg both form the canonical four-helix bundle. These conformational differences are discussed in light of the differential C5aR activation properties observed for the human and murine complement anaphylatoxins across species.

The secretory KCa1.1 channel localises to crypts of distal mouse colon: functional and molecular evidence

The colonic epithelium absorbs and secretes electrolytes and water. Ion and water absorption occurs primarily in surface cells, whereas crypt cells perform secretion. Ion transport in distal colon is regulated by aldosterone, which stimulates both Na+ absorption and K+ secretion. The electrogenic Na+ absorption is mediated by epithelial Na+ channel (ENaC) in surface cells. Previously, we identified the large conductance Ca2+-activated K+ channel, KCa1.1 or big potassium (BK) channel, as the only relevant K+ secretory pathway in mouse distal colon. The exact localisation of KCa1.1 channels along the crypt axis is, however, still controversial. The aim of this project was to further define the localisation of the KCa1.1 channel in mouse distal colonic epithelium. Through quantification of mRNA extracted from micro-dissected surface and crypt cells, we confirmed that Na+/K+/2Cl− (NKCC1) is expressed primarily in the crypts and γ-ENaC primarily in the surface cells. The KCa1.1 α-subunit mRNA was like NKCC1, mainly expressed in the crypts. The crypt to surface expression pattern of the channels and transporters was not altered when plasma aldosterone was elevated. The mRNA levels for NKCC1, γ-ENaC and KCa1.1 α-subunit were, however, under these circumstances substantially augmented (KCa1.1 α-subunit, twofold; NKCC1, twofold and ENaC, tenfold). Functionally, we show that ENaC-mediated Na+ absorption and BK channel-mediated K+ secretion are two independent processes. These findings show that KCa1.1-mediated K+ secretion mainly occurs in the crypts of the murine distal colon. This is in agreement with the general model of ion secretion being preferentially located to the crypt and not surface enterocytes.

Hyperaldosteronism after decreased renal K+ excretion in KCNMB2 knockout mice

The kidney is the primary organ ensuring K(+) homeostasis. K(+) is secreted into the urine in the distal tubule by two mechanisms: by the renal outer medullary K(+) channel (Kir1.1) and by the Ca(2+)-activated K(+) channel (KCa1.1). Here, we report a novel knockout mouse of the β2-subunit of the KCa1.1 channel (KCNMB2), which displays hyperaldosteronism after decreased renal K(+) excretion. KCNMB2(-/-) mice displayed hyperaldosteronism, normal plasma K(+) concentration, and produced dilute urine with decreased K(+) concentration. The normokalemia indicated that hyperaldosteronism did not result from primary aldosteronism. Activation of the renin-angiotensin-aldosterone system was also ruled out as renal renin mRNA expression was reduced in KCNMB2(-/-) mice. Renal K(+) excretion rates were similar in the two genotypes; however, KCNMB2(-/-) mice required elevated plasma aldosterone to achieve K(+) balance. Blockade of the mineralocorticoid receptor with eplerenone triggered mild hyperkalemia and unmasked reduced renal K(+) excretion in KCNMB2(-/-) mice. Knockout mice for the α-subunit of the KCa1.1 channel (KCNMA1(-/-) mice) have hyperaldosteronism, are hypertensive, and lack flow-induced K(+) secretion. KCNMB2(-/-) mice share the phenotypic traits of normokalemia and hyperaldosteronism with KCNMA1(-/-) mice but were normotensive and displayed intact flow-induced K(+) secretion. Despite elevated plasma aldosterone, KNCMB2(-/-) mice did not display salt-sensitive hypertension and were able to decrease plasma aldosterone on a high-Na(+) diet, although plasma aldosterone remained elevated in KCNMB2(-/-) mice. In summary, KCNMB2(-/-) mice have a reduced ability to excrete K(+) into the urine but achieve K(+) balance through an aldosterone-mediated, β2-independent mechanism. The phenotype of KCNMB2 mice was similar but milder than the phenotype of KCNMA1(-/-) mice.

Na(+) dependence of K(+) -induced natriuresis, kaliuresis and Na(+) /Cl(-) cotransporter dephosphorylation.

High dietary K+ intake is associated with protection against hypertension. In mammals, acute K+ intake induces natriuresis and kaliuresis, associated with a marked dephosphorylation of the renal Na+/Cl− cotransporter (NCC). It has been suggested that reduced activity of NCC increases the driving force for more distal tubular epithelial Na+ channel (ENaC)‐dependent K+ secretion. This study investigated the ENaC dependence of urinary K+ and Na+ excretion following acute K+ loading.

The renal and blood pressure response to low sodium diet in P2X4 receptor knockout mice

In the kidney, purinergic (P2) receptor‐mediated ATP signaling has been shown to be an important local regulator of epithelial sodium transport. Appropriate sodium regulation is crucial for blood pressure (BP) control and disturbances in sodium balance can lead to hypo‐ or hypertension. Links have already been established between P2 receptor signaling and the development of hypertension, attributed mainly to vascular and/or inflammatory effects. A transgenic mouse model with deletion of the P2X4 receptor (P2X4−/−) is known to have hypertension, which is thought to reflect endothelial dysfunction and impaired nitric oxide (NO) release. However, renal function in this model has not been characterized; moreover, studies in vitro have shown that the P2X4 receptor can regulate renal epithelial Na+ channel (ENaC) activity. Therefore, in the present study we investigated renal function and sodium handling in P2X4−/− mice, focusing on ENaC‐mediated Na+ reabsorption. We confirmed an elevated BP in P2X4−/− mice compared with wild‐type mice, but found that ENaC‐mediated Na+ reabsorption is no different from wild‐type and does not contribute to the raised BP observed in the knockout. However, when P2X4−/− mice were placed on a low sodium diet, BP normalized. Plasma aldosterone concentration tended to increase according to sodium restriction status in both genotypes; in contrast to wild‐types, P2X4−/− mice did not show an increase in functional ENaC activity. Thus, although the increased BP in P2X4−/− mice has been attributed to endothelial dysfunction and impaired NO release, there is also a sodium‐sensitive component.

Intact colonic KCa1.1 channel activity in KCNMB2 knockout mice

Mammalian potassium homeostasis results from tightly regulated renal and colonic excretion, which balances the unregulated dietary K+ intake. Colonic K+ secretion follows the pump‐leak model, in which the large conductance Ca2+‐activated K+ channel (KCa1.1) is well established as the sole, but highly regulated apical K+ conductance. The physiological importance of auxiliary β and γ subunits of the pore‐forming α‐subunit of the KCa1.1 channel is not yet fully established. This study investigates colonic K+ secretion in a global knockout mouse of the KCa1.1‐β2‐subunit (KCNMB2−/−), which apparently is the only β‐subunit of the colonic enterocyte KCa1.1 channel. We can report that: (1) Neither KCa1.1 α‐ nor the remaining β‐subunits were compensatory transcriptional regulated in colonic epithelia of KCNMB2−/− mice. (2) Colonic epithelia from KCNMB2−/− mice displayed equal basal and ATP‐induced KCa1.1‐mediated K+ conductance as compared to KCNMB2+/+. (3) K+ secretion was increased in KCNMB2−/− epithelia compared to wild‐type epithelia from animals fed an aldosterone‐inducing diet. (4) Importantly, the apical K+ conductance was abolished by the specific blocker of KCa1.1 channel iberiotoxin in both KCNMB2+/+ and KCNMB2−/− mice. Recently a novel family of auxiliary γ‐subunits of the KCa1.1 channel has been described. (5) We detected the γ1‐subunit (LRRC26) mRNA in colonic epithelia. To investigate the physiological role of the γ1‐subunit of KCa1.1 channels in colonic K+ secretion, we acquired an LRRC26 knockout mouse. (6) Unexpectedly, LRRC26 mice had a perinatal lethal phenotype, thus preventing functional measurements. On this basis we conclude that colonic K+ secretion is intact or even increased in mice lacking the β2‐subunit of KCa1.1 channel complex despite no additional compensatory induction of KCa1.1 β‐subunits.