INTRODUCTION

The fulfillment of preoperative hydration in patients who will undergo surgical trauma and therefore will be at risk of increased release of vasopressin and aldosterone, has aimed to maintain a euvolemic state, as well as ideal plasma concentrations of electrolytes such as sodium (Na⁺) and potassium (K⁺). In respect to hydration with sodium overload, it aims to inhibit the release of vasopressin and aldosterone, which can affect the body's response to surgery [1].

Concern about pre- and postoperative hypokalemic disorders in adults undergoing heart surgery has been shown in previous studies [2]. After heart surgery, the patient is usually treated with diuretics to modulation of plasma volume and pulmonary and peripheral edema [3]. Moreover, due to the association of cardiac arrhythmias with disorders of K⁺, it is necessary to perform an early diagnosis of this condition, as well as physiopathological mechanisms involved with this condition before any surgical procedure, so that therapeutic measures can be taken aiming at maintaining homeostasis [4].

The treatment of hypokalemia can be performed through the administration of K⁺ intravenously and its dilution is clinically recommended using only saline solution (SS) and not 5% glucose solution (GS 5%). The justification for this behavior is due to the possible effect that could be caused by the 5% GS, or that is, stimulation of insulin secretion that could thus interfere negatively on the quality of the replacement of K⁺². However, it is not proved that the 5% SS or GS vehicle that would dilute the K⁺ could actually interfere on the quality of this electrolyte replacement. Therefore, the data from literature seem conflicting in respect to the use of glucose and K+ in coronary artery bypass grafting [4,5].

Therefore, the aim of our study was to analyze the effects of replacement of K+ diluted in 5% SS or GS on plasma concentrations of K⁺ in rats undergone furosemide-induced hypokalemia.


METHODS

Animals

We used 24 adult Wistar rats weighing between 350 and 460 g provided by the bioterium of the ABC Medical School (FMABC). The animals used in the experiment were divided into four groups: potassium and saline solution group (K+SS; N=6); potassium and glucose solution group (K+GS; N=6); saline solution control group (SS; N=6) and 5% glucose solution control group (GS; N=6). The temperature and humidity of the bioterium were monitored and maintained around 22ºC and relative humidity of 60%, respectively. The light/dark cycle was also monitored and established as 12 hours each and before accomplishment of the experiments the animals had free access to water and food (Nuvilab). All procedures in this study were approved by the Animal Experimentation Ethics Committee of FMABC (protocol No. 003/2007).

The induction of hypokalemia was performed five days after surgery of cannulation of the femoral artery and jugular vein, then the animals were maintained for 24 hours without food and water (Nuvilab).

Cannulation of the jugular and femoral vein

During surgical procedures, the animals were anesthetized with halothane (2% mixed with 100% oxygen) administered via inhalation. The cannula implanted in the external jugular vein consisted of polyethylene tubing PE-50 (Clay Adams, NJ, USA), which is prepared so that to leave an area about 30 mm inside the blood vessel and another spare area with 70 mm at its proximal end. It was performed a melting point between these two tubes to control entry into the vascular bed. The femoral vein was cannulated using polyethylene tube PE-50 (Clay Adams, NJ, USA), 17 cm long connected to a silicone tube PE-10 (VWR International West Chester Pennsylvania, USA) of 10 cm in length . The two tubes were welded and shaped so that to present an angle of about 60° between the silicon tip and the polyethylene tip (Figure 1) to allow a better anatomical fit during implantation. In addition to that, we performed a silicone button within at ± 5mm from the melting point of the two tubes to narrow its entry into the femoral vein.

During and after the implantation of cannulae, they were maintained patent by infusing of a solution of 1UI/mL of heparin in saline solution. We used low concentration of anticoagulant to avoid the appearance of cases of pseudohyperkalemia, which could alter the results of plasma K⁺ concentrations. The permeability of the cannulae was maintained by infusion of this solution every two days. As a prophylactic measure after the surgical procedure, the animals received intramuscular injections of veterinary pentabiotic (2000 U/mL, Fort Dodge) and were maintained under observation during recovery from anesthesia.

Connector for blood collection and infusion solutions In order to not perform the infusion of replacement solutions by the same route of blood collection, it was prepared a connector with marlex mesh of 10x10mm and two stainless steel tubes (23 Gauge) with 2 mm long, twisted at an angle of 90°, which were settled on this mesh with epoxy glue sustained on the skin of the back of the animal (Figure 1). In order to not allow blood reflux when using this connector, its outlet was protected with polyethylene PE-50 (Clay Adams, NJ, USA) of 10 mm with one of its end fused.




Analysis of plasma

After 24 hours without solid food, it was performed collection of 1 mL of blood through the cannula in the jugular vein and transferred to an Eppendorf tube. The blood was centrifuged (Costar microcentrifuge) for 10 min at a speed of 1000 rpm to separate serum and plasma. After this procedure, 500 µL of plasma was pipetted and transferred to a new Eppendorf tube that was immediately sent to the Laboratory of Clinical Analysis of FMABC. The levels of plasma sodium and potassium were determined using ionselective electrode, through the equipment RAPID LAB 348-BAYER. Blood samples for analysis of [K⁺]_p were obtained in three separate stages: 1) before hypokalemic induction (control), 2) 24 h after furosemide injection and detection of hypokalemia and 3) 30 minutes after infusion of solutions replacement.

Hypokalemic induction

Five days after surgery, induction of hypokalemia was performed, using furosemide (50 mg/kg, SC). The animals were maintained in individual boxes and supplied with 200 mL of water from water fountain for 24 hours.

Replacement solutions

After detection of hypokalemia by plasma analysis, the rats were connected to the syringe that was filled with one of the following solutions: 0.4 mL of 19.1% KCl (1 mEq) + 1.6 mL of saline solution in the K+SS group; 0.4 mL of 19.1% KCl (1 mEq) + 1.6 mL of glucose solution in the K+GS group; 2 mL of saline solution in the SS group (Control) and 2 mL of 5% glucose solution in the GS group (Control). These solutions were infused into the animal with the aid of a volumetric infusion pump of SAMTRONIC ST 690 syringe scheduled in 30 minutes (0.15 mL/min).

Statistical analysis

The results were expressed as mean ± standard error of the mean and subjected to two-way Analysis of Variance (ANOVA), followed by Tukey post-test. The level of significance was accepted as P< 0.05.


RESULTS

Hypokalemic Induction

In the first pilot experiments (data not shown) performed to induce hypokalemia, it was used the dose of furosemide of 10mg/Kg. This was used in previous studies to induce sodium appetite, however, it was not found a decrease in plasma concentration of K⁺ in our pilot studies. Thus, the doses of 20 mg/kg, 30 mg/kg, 40 mg/kg and 50 mg/kg were incremented in pilot studies to assess the dose-dependent responses, being observed hypokalemia only at a dose of 50 mg/kg, which determined the dose of 50 mg/kg adopted in this study.

Analysis of plasma K⁺

Prior to subcutaneous injection of furosemide was not observed significant difference (p>0.05) of values of plasma K+ between the groups K+SS (4+0mEq/L), (4+0mEq/L), SS (4m+0Eq/L) e GS (4+0mEq/L) (Figure 2). All animals showed significant decreases (P< 0.05) of values of plasma K+ after 24 hours of administration of furosemide (K+SS =3+0mEq/ L, K+GS = 3+0mEq/L, SS = 3+0mEq/L and GS = 3+0mEq/L), and there was no significant difference (P>0.05) between groups. Shortly after 30 minutes of the K+ replacement (Figure 2), there was no significant difference between group SSK and GSK groups (P>0.05). However, there was a significant difference when comparing the values between the control SS and GS groups (P<0.05) K+SS = 4+0mEq/L, K+GS = 4+0mEq/L, SS = 3+0mEq/L and GS= 3+0mEq/L).




Analysis of plasma Na+

The concentration of plasma Na⁺ before the effects of furosemide was higher in group K+SS (P>0.05), while the GS group showed the lowest values (P>0.05) (Figure 3) (K+SS = 143,17+3mEq/L, K+GS = 142+3mEq/L, SS = 142+1mEq/L and GS = 141+1mEq/L). After 24 hours of administration of the diuretic, the K+SS showed again the highest values of plasma Na⁺ (P>0.05), while the two control groups showed the lowest values (P>0.05). It was observed signficant drop of plasma levels of Na+ in the K+SS, K+GS, SS and GS groups (P<0.05) (K+SS = 141+3mEq/L, K+GS = 140+2mEq/L, SS = 139+2mEq/L and GS = 139+2mEq/L). After 30 minutes of the K⁺ replacement (Figure 3) we found significant differences (P< 0.05) of plasma concentrations of Na⁺ between K+SS and K+GS (P< 0.05). On the other hand, no significant difference was found when comparing the values between the SS and GS control groups (P>0.05) (K+SS = 143+3mEq/L, K+GS = 140+3mEq/L, SS = 139+2mEq/ L and GS = 139+2mEq/L).




DISCUSSION

Our study examined the effect of K⁺ replacement diluted in saline solution or 5% glucose solution on plasma concentratins of Na⁺ and K⁺ in rats undergone furosemideinduced hypokalemia. It was observed that the administration of furosemide at a dose 50 mg/kg was sufficient to cause hypokalemia. Regarding the K⁺ replacement, the results obtained during this study were not different from the findings on glucose to affect the K+ replacement in cases of hypokalemia observed in humans. However, interestingly, the values of plasma Na+ after the replacement were higher in the group treated with saline solution when compared to other groups.

In order to induce hypokalemia to perform this study, it was taken as basis a model of Na+ depletion induced by treatment with furosemide (Stellar and Epstein [6], Denton [7], Grossman [8] and Fitzsimons [9]), which has been used in studies on the mechanisms of thirst and sodium appetite. The animals in our study were fed only with water supply. Studies by Pereira et al. [10] reported that the dose of 10 mg/kg (SC route) of furosemide induces a maximum and rapid loss of water and electrolytes - including Na⁺ and K⁺. Our results, however, showed no hypokalemia using this dose. Thus, this study has began in accordance with the model described, but without success. In order to induce hypokalemia in experimental study with animals, we used the principle of dose-dependence, increasing the dose of furosemide with subsequent assessment of the levels of plasma K⁺. The doses of 20, 30, 40 and 50 mg/kg (SC route) were used, and hypokalemia was observed only when we used the dose of 50 mg/kg of this drug.

We observed during the development of this experimental protocol, that after three days of treatment with subsequent doses of furosemide, the animals did not present hypokalemia, a fact that may be related to the declining of furosemide action in rats under K+ depletion, as suggested in studies of Hropot et al. [11]. In this study, it was reported that deprivation of K⁺ can reduce the binding of furosemide in its areas of action. Moreover, this phenomenon may be related to the findings of Stummer et al. [12], which suggest that the increased influx of K⁺ to the intracellular environment in acute hypokalemia is compensated by the stimulation of K+ efflux via Na⁺_-K⁺_- ATPase to restore the balance of K⁺ in cerebrospinal fluid. However, this was not observed in our study. Thus, adaptation to the doses of furosemide in our experimental conditions remains to be determined.

Since the negative consequences of potassium replacement orally in patients after CABG procedure were observed previously [3], we used the intravenous administration in our study as a proposal treatment. The results of our study showed that replacement of K⁺ diluted in saline solution and separately in glucose solution, did not change the quality of the K⁺ replacement in animals undergone furosemide-induced hypokalemia.

A study by Sweeney [13] showed that the association of insulin with glucose could therefore promote the appearance of hypokalemia. This could difficult the correction of K⁺ levels, due to the redistribution of this ion to the intracellular environment. Study by Muntzei et al. [14] showed that hyperinsulinemia increases sympathetic activity and reduces K⁺ levels in plasma, because it activates the renin-angiotensin-aldosterone system. Their study showed that hyperinsulinemia caused decreased levels of K⁺ plasma, but in our study there was no injection of insulin, which seems to be one of the hypotheses for the absence of changes in the replacement of the groups assessed.

It is noteworthy to emphasize that in humans it is not recommended to use the 5% GS vehicle during the K⁺ replacement, since glucose stimulates an increase in insulin and some authors [15-17] reported that this fact stimulates the influx of K⁺ to the intracellular invironment, suggesting a difficulty in reaching a state of normokalemia. However, this effect was not observed in experimental animals in our study. One hypothesis that could explain the noninterference of this vehicle in replacement of K⁺ is the fact that it is iso-osmotic in relation to plasma. The differences between the SS is mainly due to the osmolar gap when compared with the 5% GS. The first has its osmolarity around 308 mOsm/L while the second 252 mOsm/L, which makes it clear that, despite the difference of 46 mOsm/L, the two solutions are iso-osmotic in relation to plasma and such solutions are considered important in volume replacement in clinical practice because they do not affect the osmotic balance. Thus, further studies are needed to clarify whether there would be changes in insulin concentration and K⁺ plasma during the infusion of hyperosmolar solutions of glucose in experimental animals.

The intriguing fact that the plasma concentration of Na⁺ after the K⁺ replacement was higher in the K ⁺ SS compared with other groups suggests that the replacement of K⁺ diluted in SS in the case of hypokalemia, corroborates for maintenance of plasma levels of Na⁺, in addition to effectively restore K⁺ plasma levels.

Summarizing, according to our findings, potassium replacement in SS or 5% GS vehicles did not affect the quality of replacement of [K⁺]_p in rats with furosemideinduced hypokalemia.


ACKNOWLEDGMENTS

This study had financial support from the Center of Studies, Researches and Health Consultancy (NEPAS).