Defective small intestinal anion secretion, dipeptide absorption, and intestinal failure in suckling NBCe1-deficient mice

The electrogenic Na⁺HCO₃⁻ cotransporter NBCe1 (Slc4a4) is strongly expressed in the basolateral enterocyte membrane in a villous/surface predominant fashion. In order to better understand its physiological function in the intestine, isolated mucosae in miniaturized Ussing chambers and microdissected intestinal villi or crypts loaded with the fluorescent pH-indicator BCECF were studied from the duodenum, jejunum, and colon of 14- to 17-days-old slc4a4-deficient (KO) and WT mice. NBCe1 was active in the basal state in all intestinal segments under study, most likely to compensate for acid loads imposed upon the enterocytes. Upregulation of other basolateral base uptake mechanism occurs, but in a segment-specific fashion. Loss of NBCe1 resulted in severely impaired Cl⁻ and fluid secretory response, but not HCO₃⁻ secretory response to agonist stimulation. In addition, NBCe1 was found to be active during transport processes that load the surface enterocytes with acid, such as Slc26a3 (DRA)-mediated luminal Cl⁻/HCO₃⁻ exchange or PEPT1-mediated H⁺/dipeptide uptake. Possibly because of the high energy demand for hyperventilation in conjunction with the fluid secretory and nutrient absorptive defects and the relative scarcity of compensatory mechanisms, NBCe1-deficient mice developed progressive jejunal failure, worsening of metabolic acidosis, and death in the third week of life. Our data suggest that the electrogenic influx of base via NBCe1 maintains enterocyte anion homeostasis and pH_i control. Its loss impairs small intestinal Cl⁻ and fluid secretion as well as the neutralization of acid loads imposed on the enterocytes during nutrient and electrolyte absorption.

Sarco(endo)plasmic reticulum Ca2+ ATPase isoforms and their role in muscle physiology and pathology

Recent studies suggest that SR Ca2+ transport function is altered in hypertrophied and failing myocardium. To understand whether alterations in SR Ca2+ ATPase levels affect myocardial contractility, we generated transgenic mice that specifically overexpress SERCA2a or SERCA1 pump in the mouse heart, using the cardiac alpha-MHC promoter. Analysis of SERCA2a transgenic mice show both an increase in mRNA and protein levels (120-150% of the wild type). Isolated work performing heart preparations revealed that SERCA2a mice have improved myocardial performance. On the other hand, SERCA1 overexpression in the heart resulted in isoform replacement without any change in total SERCA protein. Interestingly, SERCA1 transgenic hearts exhibited super contractility with a significant increase in rates of muscle contraction (+dp/dt) and relaxation (-dp/dT). The time to peak pressure and half-time to relaxation were significantly shorter.

The relative teratogenic index and teratogenic potency: proposed components of developmental toxicity risk assessment

Teratogenicity tests should provide answers to three questions: (1) Can the agent induce developmental defects? ("teratogenic potential"); (2) What are the effective doses? ("teratogenic potency"); and (3) Are effective doses below adult toxic doses? ("teratogenic hazard"). The answers to (2) and (3) should be quantitative in nature, but there are no accepted parameters to express these properties. In this paper we propose parameters for the description of teratogenic potency and hazard in quantitative terms. Derivation and calculation of the parameters are illustrated by the analysis of adult lethality and teratogenicity data of eight structurally related anhydrides and imides, following testing in the CD-1 mouse. Teratogenicity was evaluated following treatment on Days 8-10 of gestation, using an average of four dose groups per compound and at least 10 dams per group. Adult lethality was estimated following a similar 3-day dosage schedule with an average of 6 dose groups per compound and at least 8 animals per group. Dose-response relationships of teratogenicity were fitted to a probit model from which tD50 (median effective dose), and other effective doses were computed. It is proposed that tD05, as a minimum teratogenic dose, best represents teratogenic potency. In this study, potency ranged from 0.17 mmol/kg/day for phenytoin to 5.2 mmol/kg/day for ethosuximide. In order to measure teratogenic hazard a ratio between adult toxic (lethality was chosen as the most appropriate measure) and teratogenic responses was made. Since the dose-response slopes of lethality and teratogenicity were different, a simple ratio between median effective doses could not be used. It is shown that a ratio of LD01 to tD05 provides a "Relative Teratogenic Index" (RTI) which reflects the teratogenic hazard of a test agent. The following RTI values (LD01/tD05) were computed in this study: phthalic anhydride, 0.9; phensuximide, 1.0; succinic anhydride, 1.0; ethosuximide, 1.2; phenytoin, 1.6; phenacemide, 3.3; trimethadione, 4.0; and sodium valproate, 4.1. For these data, tD05 and RTI clearly represent the differing teratogenic potencies and hazards of the tested compounds. it is suggested that these parameters may be useful in comparative teratogenicity studies and may be valuable components of developmental toxicity risk assessment.