UNISP

Rubidium as Substitute for Potassium  in Rat Cardiac Tissue

 

Bisola O. Asaolu and Mai T. Vue

Department of Biological Sciences

 

Minnesota State University, Mankato. USA.

 

 

Abstract

The purpose of this project is to keep studying the sodium/potassium ATPase pump in the cells of rat hearts. A normal cell contains a sodium/potassium ATPase pump that functions to maintain a high intracellular level of potassium (K+) and high extracellular levels of sodium (Na+). This project uses rubidium as a marker for the activity of the pump mechanism. Since rubidium (Rb+) shares similar chemical properties with potassium and has a similar atomic radius, it can replace potassium in all known metabolic processes (Olga Jilkina et al). We used rat hearts as a model to determine and quantify how rubidium will take over as potassium in the sodium/potassium pump.   The Langendorff System was used for these experiments, and in-vitro heart beats were obtained for an average of 40 minutes. Rubidium was measured using the Energy-Dispersive X-Ray Spectroscopy system (EDS) associated with the SEM (JEOL JSM-6510LV). An uptake of rubidium was observed with each heart perfused with rubidium chloride, and rubidium uptake (same for potassium) was recorded as long as the hearts continued to beat. Our results suggest that rubidium was taken up by the cells of the heart in a positive slope with respect to time.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Background/Introduction

The Na+/K+ ATPase pump plays an important role in the sympathetic fight-or-flight response in cardiovascular tissue (Bers and Vespa, 2009). This pump is an active transporter in the cells, which means it requires energy (ATP) and moves Na+ and K+ ions against their concentration gradient (Na+ out the cell and K+ into of the cell). Previous research with NMR (Nuclear Magnetic Resonance) has used rubidium as a marker for the activity of potassium because of the strong sensitivity of the 87Rb isotope in comparison to the 39K isotope. This study demonstrates Rb as a congener of K in that they have a similar hydrated ionic radius (K+ = 2.76Å and Rb+ = 2.93Å) (Bers and Vespa, 2009). The location (same column) of these two elements on the periodic table also shows that they share similar chemical properties. Energy Dispersive (X-Ray) Spectroscopy is used to determine the weight percent of elements in a specific area. The electron beam submits electrons in the specimen, some of which are emitted as characteristic x-rays. A Silicon Drift Detector (SDD) is then used to characterize each element present in the sample area by element specific electron spectrums.

 

 

Methods

Laboratory rats were anesthetized with isoflurane gas. While under anesthesia, heparin (0.1 ml) was injected into the descending aorta as an anticoagulant. The hearts were then removed, placed in ice cold 0.9% saline solution, and a cannula placed in the aorta of each heart. The heart was infused with a buffer solution containing rubidium chloride or potassium chloride as necessary so that it would circulate through the coronary arteries using the Langendorff system. The Langendorff apparatus was used as a means to control pH, temperature, and pressure of the solutions perfusing the heart. All heart tissues were bubbled with oxygen gas (O2) throughout the duration of the experiments. At the end of each procedure, each heart was freeze-dried to prevent diffusion from occurring, thereby causing a change in the experiment. Using the Scanning Electron Microscopy (SEM), the weight percent of specific elements including rubidium and potassium was measured in the cardiac tissue to determine the amount that was absorbed into the cells.  Rubidium and potassium uptake were measured using the Energy-Dispersive X-Ray Spectroscopy system (EDS) associated with the SEM. This system allows for element specific spectrum x-rays to be measured, in this case the elements of Rb and K.

 

 

Results

The results of this research show an increase in the average weight percent of Rubidium in Rubidium treated hearts versus control hearts. There was no significant difference between the uptake of potassium and rubidium. Also, there was no significant difference in the regional uptake of rubidium in the cardiac tissue, although a trend was observed.

 

 

Discussion

Rubidium has been used in previous research studies as a marker for the activity of the Na/K ATPase pump specifically in cardiac tissue. Infusion of rubidium did not show toxic effects at K uptake levels of 30% in rats and dogs and 12% in humans (Meltzer, 1991), which supports the suggestion that Rb may be an enhancement to microtomography scanning (CT Scanning). Rb could be detected by CT Scanning in order to determine the activity, (or lack thereof) in the Na+/K+ ATPase pump.

The ultimate goal of this research is to determine if Rubidium can substitute perfectly for Potassium in the Na/K ATPase pump.  Our results show that Rubidium uptake increases with increase in time, but so far has not reached an asymptotic value. This infers that rubidium does not only substitute for potassium but also competes well with potassium in the Na/K ATPase Pump. The regional trends in the levels of Rubidium uptake might be explained through the anatomical differences in the right and left coronary arteries. The right coronary artery covers a larger fraction of the heart than the left coronary artery does, thereby delivering more blood/buffer to the right side of the heart. In spite of the remarkable uptake of rubidium, our data also suggests that the cardiac tissue has a higher affinity for Potassium when compared to rubidium.

 

 

Future Work

Future research on this project would include the addition of the Ouabain to the solution used in perfusing the rat cardiac tissue as an inhibitor of the Na+/K+ ATPase pump. This would be done with the expectation that sodium levels would increase inside the cells and the amount of Rb would decrease because the pump would no longer be activated (Na+/K+ - pump inhibition).

 

 

 

References

Bers, D. M., Vespa, S. (2009). Na/K-ATPase – an integral player in the adrenergic fight-or-flight response. Trends Cardiovasc Med, 19, 111-118.

Kupriyanov, V. V., Xiang, B., Sun, J., Jilkina, O. (2002). The effects of drugs modulating K+ transport on Rb+ uptake and distribution in pig hearts following regional ischemia: 87Rb MRI study. NMR Biomed, 15, 348-355.

Meltzer, H. L. (1991). A pharmacokinetic analysis of long-term administration of rubidium chloride. J. Clin. Pharmac., 31, 179-184.