Sexual signals are considered costly to produce and maintain under the handicap paradigm, and the reliability of signals is in turn thought to be maintained by these costs. Although previous studies have investigated the costly nature of signal production, few have considered whether honesty might be maintained not by the costliness of the signal itself, but by the costs involved in producing the signalled trait. If such a trait is itself costly to produce, then the burden of energetic investment may fall disproportionately on that trait, in addition to any costs of signal maintenance that may also be operating. Under limited resource conditions, these costs may therefore be great enough to disrupt an otherwise reliable signal-to-trait relationship. We present experimental evidence showing that dietary restriction decouples the otherwise honest relationship between a signal (dewlap size) and a whole-organism performance trait (bite force) in young adult male Anolis carolinensis lizards. Specifically, while investment in dewlap size is sustained under low-resource condition relative to the high-resource treatment, investment in bite force is substantially lower. Disruption of the otherwise honest dewlap size to bite force relationship is therefore driven by costs associated with the expression of performance rather than the costs of signal production in A. carolinensis .

Recent anatomic findings indicate that in the upper inner medulla of the rodent kidney, tubules, and vessels are organized around clusters of collecting ducts (CDs). Within CD clusters, CDs and some of the ascending vasa recta (AVR) and ascending thin limbs (ATLs), when viewed in transverse sections, form interstitial nodal spaces, which are arrayed at structured intervals throughout the inner medulla. These spaces, or microdomains, are bordered on one side by a single CD, on the opposite side by one or more ATLs, and on the other two sides by AVR. To study the interactions among these CDs, ATLs, and AVR, we have developed a mathematical compartment model, which simulates steady-state solute exchange through the microdomain at a given inner medullary level. Fluid in all compartments contains Na(+), Cl(-), urea and, in the microdomain, negative fixed charges that represent macromolecules (e.g., hyaluronan) balanced by Na(+). Fluid entry into AVR is assumed to be driven by hydraulic and oncotic pressures. Model results suggest that the isolated microdomains facilitate solute and fluid mixing among the CDs, ATLs, and AVR, promote water withdrawal from CDs, and consequently may play an important role in generating the inner medullary osmotic gradient.

Every collecting duct (CD) of the rat inner medulla is uniformly surrounded by about four abutting ascending vasa recta (AVR) running parallel to it. One or two ascending thin limbs (ATLs) lie between and parallel to each abutting AVR pair, opposite the CD. These structures form boundaries of axially running interstitial compartments. Viewed in transverse sections, these compartments appear as four interstitial nodal spaces (INSs) positioned symmetrically around each CD. The axially running compartments are segmented by interstitial cells spaced at regular intervals. The pairing of ATLs and CDs bounded by an abundant supply of AVR carrying reabsorbed water, NaCl, and urea make a strong argument that the mixing of NaCl and urea within the INSs and countercurrent flows play a critical role in generating the inner medullary osmotic gradient. The results of this study fully support that hypothesis. We quantified interactions of all structures comprising INSs along the corticopapillary axis for two rodent species, the Munich-Wistar rat and the kangaroo rat. The results showed remarkable similarities in the configurations of INSs, suggesting that the structural arrangement of INSs is a highly conserved architecture that plays a fundamental role in renal function. The number density of INSs along the corticopapillary axis directly correlated with a loop population that declines exponentially with distance below the outer medullary-inner medullary boundary. The axial configurations were consistent with discrete association between near-bend loop segments and INSs and with upper loop segments lying distant from INSs.

Our goal is to determine if AQP2 expression patterns in the renal medullary collecting duct (CD) contribute to the ability of the kangaroo rat to produce a more highly concentrated urine than that of the Munich‐Wistar rat. Inner medullas from kidneys of moderately concentrating animals were embedded in Spurrs and 1 μm thick transverse sections cut at 500 μm intervals throughout the corticopapillary axis and labeled for AQP2. Apical and basal plasma membrane AQP2 expression levels were quantified on a pixel‐by‐pixel basis in each CD in 0.11 mm2 sections. The basal/apical AQP2 expression ratio at 1000 μm below the outer medulla is nearly equal in both species (~0.50), whereas at 4500 μm below the outer medulla, the basal/apical AQP2 expression ratio is greater in the CD of kangaroo rat (~1.2) relative to Munich‐Wistar rat (~0.70). Prior studies have shown that transepithelial water permeability of the inner two‐thirds of Sprague‐Dawley rat isolated perfused inner medullary CDs markedly exceeds that of the outer third without vasopressin. The high water permeable segment correlates with high basal/apical AQP2 expression ratios. A heightened basal/apical AQP2 expression ratio in kangaroo rat CD is consistent with prior studies indicating peritubular hypertonicity is a regulatory factor of CD water permeability and basal/apical AQP2 expression ratios. NIDDK DK083338, NSF IOS095285, APS/NIDDK STEP‐UP, WAESO.

Recent findings in the upper inner medulla of the rat kidney indicate thin limbs of Henle's loops and vasa recta are arranged in repeating patterns among clusters of collecting ducts (CDs). Within CD clusters an interstitial compartment (interstitial nodal space INS) can be seen in transverse sections of the inner medulla. INSs are delimited by a single CD, one or more ascending thin limbs (ATLs) and ascending vasa recta (AVR). In this study we investigated possible fluid and solute flows among CDs, ATLs and AVR, by examining the spatial interactions of structures that form the INSs. Nephron tubules and vasa recta were identified using immunohistochemistry. Spatial interactions were assessed by quantifying lengths of abutting segments of the rat inner medulla. INS architecture was incorporated into a mathematical model simulating steady‐state solute exchange through INSs at given inner medullary levels. At 400 μm below the outer medullary‐inner medullary border approximately 70% of CD was found to abut the INS. In the INS there are negative fixed charges representing macromolecules, e.g. hyaluronan, balanced by Na+ that may promote water reabsorption from CDs. Model results quantify potential fluid and solute fluxes between INSs, nephrons and vessels, and support the hypothesis that INSs serve as fluid and solute mixing chambers that support the inner medullary osmotic gradient.NIH: DK083338 and NSF: DMS0701412