Effect of Potassium and Calcium Ions on Heart Function
In the discussion of membrane potentials in Chapter 5, it was pointed out that potassium ions have a marked effect on membrane potentials, and in Chapter 6 it was noted that calcium ions play an especially important role in activating the muscle contractile process. Therefore, it is to be expected that the concentration of each of these two ions in the extracellular fluids should also have important effects on cardiac pumping.
Effect of Potassium Ions. Excess potassium in the extracellular fluids causes the heart to become dilated and flaccid and also slows the heart rate. Large quantities also can block conduction of the cardiac impulse from the atria to the ventricles through the A-V bundle. Elevation of potassium concentration to only 8 to 12 mEq/L—two to three times the normal value—can cause such weakness of the heart and abnormal rhythm that this can cause death.
These effects result partially from the fact that a high potassium concentration in the extracellular fluids decreases the resting membrane potential in the cardiac muscle fibers, as explained in Chapter 5. As the membrane potential decreases, the intensity of the action potential also decreases, which makes contraction of the heart progressively weaker.
Effect of Calcium Ions. An excess of calcium ions causes effects almost exactly opposite to those of potassium ions, causing the heart to go toward spastic contraction. This is caused by a direct effect of calcium ions to initiate the cardiac contractile process, as explained earlier in the chapter.
Conversely, deficiency of calcium ions causes cardiac flaccidity, similar to the effect of high potassium. Fortunately, however, calcium ion levels in the blood normally are regulated within a very narrow range. Therefore, cardiac effects of abnormal calcium concentrations are seldom of clinical concern.
Growing Conditions Affecting Potassium Availability
Potassium (K) is one of the three primary macronutrients that all plants need to mine from the soil that is necessary for proper growth. On a fertilizer grade analysis, it is the third number listed (in the form of potash) behind phosphorus and nitrogen; yet, potassium is sometimes overlooked as an important plant nutrient until problems arise.
This year, potassium deficiency symptoms appear to be occurring more than in years past. The growing conditions that we have experienced this year have contributed to potassium deficiency. One obvious factor that affects potassium deficiency is low soil K; if there’s little to no K in the soil, plants will show deficiencies. However, K deficiency can appear even in soils that have adequate levels of soil K if certain criteria are met.
Potassium deficiency tends to show up and/or become exaggerated during periods of drought and in compacted fields. Under drought and compacted soil scenarios, plants have smaller, restricted root systems that cannot grow deep enough to access soil K located deeper in the soil profile. Planting into wet fields in the spring can cause soil compaction and glazed sidewalls, leading to K deficiency if dry conditions manifest later in the season. This is exactly what we’ve seen this year in Maryland. Potassium deficiencies do not often appear early in the season, especially in corn. Symptoms typically begin to appear a few weeks after planting, and in corn, begins to appear during jointing.
Potassium deficiency starts out as chlorosis (yellowing) of the leaf margins (edges), progressing down the leaf blade with the midrib staying green (Figure 1). Eventually the tip will begin to die, advancing down the leaf until it is dead. This is opposite of nitrogen deficiency symptoms, which starts as chlorosis in the middle of the leaf and progresses towards the margins. Potassium is mobile in the plant; meaning that plants can sacrifice K in the lower leaves and move it up to the growing point (newly emerging leaves) to support growth. Therefore, symptoms of K deficiency appear first in the lower leaves and progress up the plant as it grows taller.
Figure 1. Potassium deficiency in corn. Notice chlorosis starting on the lower leaves on leaf margins progressing towards the midrib with tip and margin dieback.
Managing a potassium deficiency mid-season can be achieved by topdressing with a K fertilizer if the plants are still small enough. Keep in mind that a granular K fertilizer will need to be dissolved and soaked down into the soil profile, so a mid-season application will only help if it rains or you irrigate following application. Successful management of K should start with your soil test. Test your field to see how much K you have and if you’re low, then fertilize accordingly. Avoid practices that can cause soil compaction, such as working or planting wet ground. If you have compacted soils, consider subsoiling in the fall or planting deep-rooted, diakon-type radishes to shatter the hardpan. Control weeds, as they can compete with your crop for potassium.
Most crops require a large amount of potassium; roughly the same or more than nitrogen. Keep in mind that forage crops, including hay, remove substantial amounts of potassium from the soil (upwards of 200 lbs/acre/yr), so it is necessary to fertilize with K to replace what’s been removed, especially if you plant behind a hay or forage crop.
A well-balanced diet is the key to balanced potassium levels
Working to carry crucial signals to heart, nerve and muscle cells, potassium plays a key role in overall health and function. When the body’s supply of this vital mineral and electrolyte runs too low or too high, one may be at risk for medical issues that include digestive distress, abnormal heart rhythms and potential cardiac arrest.
“Potassium is an electrolyte that helps conduct electricity in the body. It’s crucial to our heart, helping trigger it to squeeze blood through our body,” said Stephen Compston, a licensed and registered dietitian with Renown Health. “Potassium is also crucial for our muscles and skeletal muscles — helping with movement — and the muscles in our digestive system, helping with contraction, which is important for digestion.”
People whose potassium levels are too high or too low typically suffer from medical conditions or are on medications that affect the regulation or absorption of this mineral within the body. For example, people with chronic kidney disease may have too much potassium in their bloodstream.
“Our kidneys work to regulate levels of potassium in our blood, so people who have any disorders or disease related to kidney function need to be careful with their potassium intake,” said Johanna Dibble, a licensed and registered dietitian with Northern Nevada Medical Center. “People who have malabsorption disorders or other states of malnutrition, along with athletes who sweat excessively, are at risk for low potassium levels.”
As far as medications that may affect potassium levels, these include ACE inhibitors and diuretics, among others. If you happen to have a medical condition or take a medication that could affect your potassium level, your doctor can monitor your body’s supply of this mineral through a basic blood test.
“Low potassium levels in the blood, also known as hypokalemia, can be asymptomatic or can cause GI distress, weakness and fatigue, tingling or numbness, heart palpitations, frequent urination, and nausea or vomiting,” Dibble said. “High potassium levels in the blood, also known as hyperkalemia, share the same set of symptoms as hypokalemia. More serious symptoms include a slower heart rate or even sudden cardiac arrest.”
For folks who do not have health problems or prescriptions that could have an impact on potassium levels, maintaining an adequate supply of this electrolyte tends to come down to a well-balanced diet. Experts warn against seeking out a potassium supplement without physician supervision.
“Most healthy adults can get enough potassium through diet alone,” Dibble said. “Do not take potassium supplements unless directed by a doctor, as having too high potassium can be dangerous and even life threatening.”
By filling your plate with a variety of unprocessed foods, chances are good you’ll be boosting your body’s supply of potassium. Prime sources of potassium include potatoes, broccoli, bananas, cantaloupe, avocado, salmon, beans, yogurt and milk.
“As with many things health-related, a diet packed with whole, plant-based foods is best. When you’re eating these foods, you’re not only getting potassium, but also essential fiber, vitamins and minerals,” Compston said. “Most of us don’t get enough potassium in our diet because we aren’t eating enough unprocessed foods.”
Watch for K Deficiency
As the crops are finally getting going, potassium deficiency is showing up in some fields. Potassium deficiency symptoms occur on the edges of the lower leaves on most plants.
In corn and soybeans, the symptoms start as yellowing on the edges of the lower leaves. Eventually the edges of the leaves will turn brown and the whole leaf may die. In alfalfa, the symptoms start as small spots on the edges of the leaves which eventually grow together and end up as yellow and finally brown leaf edges. As the deficiency becomes more severe, the symptoms will progress higher up the plant.
Plants take up about the same amount of K as they do N. Also note that removal of K depends on how the crop is harvested. In spite of the large uptake of K, relatively small amounts of K are removed in grain. However, when the whole plant is harvested as silage and hay crops, a very large amount of K is removed. For example corn grain at 200 bu/A removes 60 lb K2O/A, but if the same corn crop is harvested for silage, the removal is 240 lb K2O/A. This can make a big difference in the K levels in different field on a farm. Many fields that get a lot of manure in the corn part of the rotation have good reserves of potassium. Fields receiving little manure or that are late in the forage rotation can have fairly low soil test levels and fields coming out of forages often are lowest in K.
As always, begin with a soil test. Even though it maybe the last minute, the Ag Analytical Services Lab at Penn State normally has about 2 day turn-around with soil samples. So if you overnight your samples to the lab and have signed up for free web access to your results you can still have your results in time to make topdressing decisions. Other labs have similar service. For forage crops low in K, fertilize between cuttings. If you have a forage field with K deficiency, it is especially important to correct this before going into the winter. Potassium is very important for winter survival. For corn or soybeans, if the soil test is low and/or the crop is showing deficiency symptoms, topdressing with potash will be beneficial. Because K is not very mobile, this topdressing will not usually give as good a result as if the soil test was optimum or the K was applied preplant, but it will help. Since the soil needs a K application at some point anyway, if it is practical, you may as well apply it now and get what you can out of it for the current crop and get a start on building K for the following crop.
One thing to keep in mind this year is that wet conditions at planting can result in sidewall compaction which can produce severe K deficiency. If the young plants are showing K deficiency, but the soil tests are optimum or better, this could be the problem. If this is the case adding K, may not provide any benefit. Hopefully, the roots will eventually get past the sidewall compaction and get the K that is already available in the soil.
Low Potassium Levels Linked to Increased Risk of Chronic Kidney Disease
New research determined a link between potassium levels, with the risk of developing chronic kidney disease in a mostly white population.
Potassium is a mineral that aids in proper bodily functioning. Levels of potassium are tightly regulated by the kidneys to help control blood pressure. In chronic kidney disease, a condition that results from kidney malfunction, patients may suffer from unregulated potassium levels in the blood. Hypokalemia is a condition when potassium levels are low (<3.5mmol/L) and hyperkalemia (>5.0 mmol/L) results from increased levels of potassium. Chronic kidney disease (CKD) patients with either hypo or hyperkalemia have a higher risk of mortality, heart attack, and hospitalization. Previous kidney studies have also found that hypokalemia may lead to kidney damage. However, the evidence demonstrating the association of hypokalemia with risk of developing CKD is not yet established.
In a recent study published in Plos One, researchers conducted a prospective study to examine the association between potassium blood level and the risk for developing CKD in predominantly white population. The study enrolled 6,000 participants with a urinary albumin concentration of 10 mg/L, a metric for hypertension and risk for CKD. Furthermore, the study also enrolled 2,592 participants with <10mg/L urinary albumin concentration. Participants who were diabetic, who already suffered from CKD and who were pregnant were excluded, leaving 5,130 participants. Circulating potassium levels, glomerular filtration rate (GFR), serum creatinine, and cystatin C were measured. Hypokalemia was defined as <3.5 mmol/L, normokalemia at 4-4.4 mmol/L, and hyperkalemia at a concentration equal to or greater than 5.0 mmol/L. CKD was determined by low GFR (<60ml/min per 1.73m2) and/or low urinary albumin excretion (UAE) (> 30mg/24h).
The researchers found mean plasma potassium levels of 4.4 mmol/L across the 5,130 participants. Hypokalemia had a low prevalence at 0.5% while hyperkalemia was slightly more common with a3.8% prevalence. Interestingly, participants with hypokalemia do not consume alcohol or smoke. They were also likely to be older, less educated and have high blood pressure as well as likely users of beta blockers and diuretics. In contrast, participants with hyperkalemia were likely to be male, to smoke, and to be White. They also have a higher UAE and non-usage of diuretics.
With regards to hypokalemia and risk of CKD, with a median follow-up of 10.3 years, researchers found that 753 participants eventually developed CKD. Participants with hypokalemia were about 5 times likely to develop CKD than those with normal potassium levels, and the risk further increased in participants who used diuretics. Participants with hyperkalemia, however, were not likely to develop CKD. Furthermore, the link between potassium levels with the risk of developing CKD changed when subjects used diuretics. In non-hypokalemic participants who used diuretics, researchers found an increased risk of CKD. Overall, the researchers concluded that hypokalemia was associated with a higher risk of CKD regardless of use of diuretics. The precise mechanism as to how hypokalemia induces kidney damage remains unclear. The current study could not yet be generalized to a broader population due to a lack of diversity in the participants. It remains to be seen whether a similar association between hypokalemia and CKD would be observed for other patients from other racial ethnicities.