Cardiac Morphology and Function, and Blood Gas Transport in Aquaporin-1 Knockout Mice

Abstract

We have studied cardiac and respiratory functions of aquaporin-1-deficient mice by the Pressure-Volume-loop technique and by blood gas analysis. In addition, the morphological properties of the animals' hearts were analyzed. In anesthesia under maximal dobutamine stimulation, the mice exhibit a moderately elevated heart rate of < 600 min(-1) and an O2 consumption of ~0.6 ml/min/g, which is about twice the basal rate. In this state, which is similar to the resting state of the conscious animal, all cardiac functions including stroke volume and cardiac output exhibited resting values and were identical between deficient and wildtype animals. Likewise, pulmonary and peripheral exchange of O2 and CO2 were normal. In contrast, several morphological parameters of the heart tissue of deficient mice were altered: (1) left ventricular wall thickness was reduced by 12%, (2) left ventricular mass, normalized to tibia length, was reduced by 10-20%, (3) cardiac muscle fiber cross sectional area was decreased by 17%, and (4) capillary density was diminished by 10%. As the P-V-loop technique yielded normal end-diastolic and end-systolic left ventricular volumes, the deficient hearts are characterized by thin ventricular walls in combination with normal intraventricular volumes. The aquaporin-1-deficient heart thus seems to be at a disadvantage compared to the wild-type heart by a reduced left-ventricular wall thickness and an increased diffusion distance between blood capillaries and muscle mitochondria. While under the present quasi-resting conditions these morphological alterations have no consequences for cardiac function, we expect that the deficient hearts will show a reduced maximal cardiac output.

Keywords: Pressure-Volume-loop technique; aquaporin-1; blood gases; heart morphology; knockout mice.

Figure 1

Body weights (A), lung weights (B), and left ventricular (LV) weights normalized for tibia length (C), for the animals used in the P-V loop measurements. Sexes are considered separately, and body weights as well as LV weights show significant differences between WT and KO animals for each sex. In the case of LV weights, the significance remains, when sexes are combined. n = 7 for both sexes together. ^*indicates statistically significant difference between KO and WT values.

Figure 2

Heart cross sections, taken perpendicular to the longitudinal heart axis, and stained with Masson's trichrome staining. Animals are those used in the P-V loop measurements. Bar 1 mm.

Figure 3

Sections prepared from cardiac tissue taken from the animals used for PV-Loop measurements. (A,B) myofiber staining, (C,D) capillary staining, (E,F) overlay of A,C, and B,D, respectively. A,C,E are from WT hearts, B,D,F are from AQP1- KO hearts. Bar length 50 μm.

Figure 4

(A) Results from PV loop measurements, averaged from seven hearts from both female and male sexes. SV, stroke volume; HR, heart rate; CO, cardia output; Ves, endsystolic volume; Ved, enddiastolic volume; EF, ejection fraction. Infusion of dobutamine was increased stepwise form 0 to 40 ng/g/min. Two-way repeated measures ANOVA indicates no significant difference between curves, indicating that all parameters of WT and KO behave identically. (B) Results from PV loop measurements, averaged from seven hearts from both female and male sexes. Pes, end-systolic pressure; Ped, end-diastolic pressure; dP/dt, maximal rate of increase of LV pressure; dP/dt, maximal rate of relaxation of LV pressure. All pairs of curves are not significantly different when tested as described in panel (A).

Figure 5

Analyses of transcripts from LV tissue of the WT and KO mice used in PV loop measurements. (A), mRNAs from two HIF-dependent enzymes GAPDH and PDK1; (B), sarco-endoplasmic reticulum Ca⁺⁺-ATPase; (C), α- and β-myosin heavy chain mRNAs.