We have already discussed the negative feedback loops in the body that are responsible for controlling the release of anti-diuretic hormone (ADH) and aldosterone in response to sodium (Na+) potassium (K+), and water levels in the bloodstream. Table 5 is a summary of these important fluid and chemical regulatory mechanisms. Although we will not be looking at Dr. Leach's space flight data about potassium, we will look at sodium data, as well as ADH and aldosterone data to determine how space flight affects the way that ADH and aldosterone are released in response to Na+ levels. Does the negative feedback loop relating these three important chemicals operate in the same way in space as it does on Earth? Let's find out.
Dr. Leach and her colleagues measured a variety of chemical constituents in the blood and urine samples that were collected daily from the astronauts. In order to be able to understand and relate the measurements to each other, it's important to know essentially everything that was going into the astronaut's body (and for that matter, almost everything that was leaving the body). Therefore, to support Dr. Leach's experiment, daily diet and fluid intake logs were maintained by each of the astronauts. In fact, before the mission, the astronauts and the scientists planned for a diet that would meet all of the Recommended Daily Allowances (RDA) of the nutrients that our bodies need. (Most of you have heard of RDAs on some cereal commercial at one time or another!) Of course, each astronaut selects what they want from the meal developed for the shuttle. Each item on this list has been analyzed for its chemical and nutritional content. From these logs and from the blood and urine samples taken daily, Dr. Leach was able to determine: (1) the daily level of sodium intake, (2) the daily plasma sodium level, and (3) the sodium level in the urine that was excreted daily.
|Figure 17. Inflight and postflight sodium intake and urinary excretion levels as a percentage of preflight levels.|
The role of the kidney in sodium excretion is quite a delicate one. On Earth, the large amount of glomerular filtrate that is formed per day (180 liters) contains a total of about 26,000 milliequivalents (mEq) of salt. The average intake of sodium per day is only 150 mEq. Therefore, in order to maintain a tight equilibrium of sodium content in the body, the kidneys can be allowed to excrete only about 150 of the 26,000 mEq. Consequently, the principal role of the kidney tubules in sodium excretion is to reabsorb sodium back into the bloodstream, not to excrete it. Furthermore, it must reabsorb just exactly the right amount of sodium, about 99.3% of the total, and must adjust the final amount of sodium that flows into the urine hour by hour or day by day so that it will balance the daily intake of sodium. Thus, the two jobs that the kidney's tubular system must perform for sodium excretion are: (1 ) to reabsorb nearly all sodium back into the bloodstream, and (2) to adjust the remaining amount that is excreted very carefully so that an absolute balance can occur between sodium intake and sodium excretion. Is this balance maintained in space? Well, we are about to find out.
For Dr. Leach's experiment, sodium intake was measured to be about 140 mEq per day before flight. The results shown in Figure 17 reflect the percent change from that value for the inflight and postflight periods. Both sodium intake levels and the level of sodium excretion in the urine are shown and, as you can see, sodium intake increased during flight except on the first and tenth days. Urinary excretion of sodium was elevated by an even greater percentage, but until FD12, intake and excretion behaved in an almost parallel fashion. This indicates that intake and excretion remained essentially in balance. But what happened on FD12 and after landing (R+0 through R+15)? A complete explanation has yet to be developed for these "special" periods, but you can bet that the system was working to reestablish an "Earth-normal" condition after the space part of the mission was completed. Remember, too, that the astronauts fluid loaded on the last day inflight. Thus, some of the data beyond that point were influenced by this rapid ingestion of saline.
We have just been looking at the results of measurements made of levels of sodium intake and of urinary excretion of sodium. The urinary sodium data gave an indication of how much sodium was not reabsorbed from the kidneys back into the bloodstream. Now let's look at how well the sodium levels in the bloodstream were regulated by looking at the results of Dr. Leach's analysis of plasma sodium levels. From Figure 18, you can see that, although the plasma sodium levels were elevated compared to preflight values, their difference was never more than about 2%. The initial increase, shown on FD1, was a measurement made just 3.5 hours after launch; this increase may be due to the fact that the astronauts were eliminating fluid in response to the "flood" into the upper part of the body. The body may have been reabsorbing more sodium into the bloodstream to slow the elimination of fluids. For the remainder of the flight, plasma sodium was increased less than 1% from preflight values.
Table 5. A summary of hormone responses to sodium and potassium levels in the bloodstream. What is detected in the bloodstream   Endocrine Response   Renal Response   Result High concentration of Na+ in the bloodstream Posterior pituitary gland releases ADH Kidneys increase reabsorbtion of water in to bloodstream to dilute Na+. Less urine is produced High condentration of K+ in the bloodstream Adrenal cortex gland releases aldosterone Kidneys stop reabsorbing and begin excreting K+. More K+ excreted in the urine Low concentration of Na+ in the bloodstream Adrenal cortex gland released aldosterone Kidneys increase reabsorption of Na+ back into bloodstream to preserve plasma Na+. Less Na+ excreted in the urine
|Figure 18. Inflight and postflight plasma sodium changes compared to preflight values.|
Now, let's look at the results of Dr. Leach's measurement of ADH and aldosterone. Keep the sodium data in mind as we attempt to connect the inflight and postflight hormone levels with those of the sodium levels. The question is, can we predict how ADH and aldosterone levels will behave? Before we look at Dr. Leach's space flight data for ADH and aldosterone, let's first examine how hormones are measured.
As powerful and important as hormones are to the body, most hormones are present in the blood in incredibly tiny quantities, some in concentrations as low as one-millionth of a milligram (1 nanogram or 10 grams) per milliliter of blood. This is an amazingly small amount! Therefore, except in a few instances, it has been almost impossible to measure these concentrations by any of the usual chemical means. Fortunately, though, an extremely sensitive method, known as a radioimmunoassay technique, was developed in the late 1960s that revolutionized the measurement of hormones. Let's take this word apart to get a better idea of what is involved in the method. This is often helpful if you are faced with what seems to be a very complicated term. Don't let it scare you. Rip it apart and then put it back together so that it makes sense!
The first part, radio-, usually refers to radioactivity. The second part, immuno-, usually refers to something related to the immune system. In this case, it refers to the use of antibodies that are highly specific for the hormone of interest. And the third part, -assay, is defined in the dictionary as "to analyze for one or more specific components." So, let's see if we can put it all together. The radioimmunoassay technique is an analysis technique (assay) that uses antibodies (immuno) and radioactivity (radio). Actually, we are right! Let's talk about the radioimmunoassay approach to the measurement of hormones. In particular, let's see how ADH levels are measured.
First of all, a synthetic antibody (synthetic = artificially produced) is developed in the laboratory that is able to bind together with the ADH. The role of natural antibodies that circulate in our bloodstream is to "capture" and bind with viruses, bacteria, and other foreign cells in the body so that the cells cannot cause the damage that they otherwise would. It is the antibody's binding capability that is their strength. In the case of the radioimmunoassay technique, the synthetic antibodies are developed and used for their binding capability as part of the technique to measure hormone levels.
A plasma (or urine) sample from the astronaut that contains ADH is mixed with the antibodies. In addition, a known amount of "standard" ADH that has been tagged with radioactivity is added to the antibody mixture. The "standard" ADH is something that is prepared in the laboratory to serve as a reference, something that has already been measured and that can be used to compare the unknown quantity of natural ADH. The radioactive tag is added to the standard ADH so that it can be distinguished from the natural ADH that is coming from the astronaut's plasma. As the antibodies, the standard radioactive ADH, and the astronaut's plasma are mixed, one main requirement must be met: there must be too little antibody to bind completely to both the radioactive hormone and the natural hormone in the fluid to be assayed. Therefore, the natural hormone in the plasma and the radioactive standard hormone compete for the binding sites on the antibody (Figure 19). The quality of each hormone that binds to the antibodies reflects the concentration of the hormones in the mixture.
|Figure 19. The radioimmunoassay technique to measure hormone levels in the body involves mixing natural hormones (from a blood sample) and radioactively prepared hormones together, and adding that mixture to a solution containing artificially produced antibodies. The two hormones compete for binding sites on the antibodies. The hormones attach to the antibodies in amounts that ore proportional to their concentration.|
After binding has reached equilibrium, the antibody-hormone complex is separated from the remainder of the solution, and the quantity of radioactive hormone bound with antibody is measured by radioactive counting techniques. We have already discussed this technique in the previous chapter. If a large amount of radioactive ADH has bound with the antibody, then it is clear that there was only a small amount of natural ADH to compete with the radioactive hormone, and therefore the concentration of the natural ADH in the assayed fluid was small. Conversely, if only a small amount of radioactive ADH has bound, it is clear that there was a very large amount of natural ADH to compete for the binding sites. It is not enough, though, to say that a "small amount" or a "large amount" of natural ADH is present. The final step in the technique is to quantify (to determine the actual quantity of) the ADH level.
Quantifying the data is done by performing the radioimmunoassay technique on a known "standard" amount of untagged ADH at different concentration levels and plotting the data to establish a "standard curve" on a graph. By comparing the radioactive counts recorded from the original assay procedure with the standard curve, one can determine within an error of + 10 to 15% the concentration of ADH in the assayed fluid. Of course, other hormones besides ADH can be measured this way. In fact, as little as one-trillionth of a gram of hormone is often assayed in this way! Dr. Leach employed this technique not only for the measurement of ADH, but also for aldosterone. Let's look at the data.
|Figure 20. Inflight and postflight plasma ADH changes compared with preflight values.|
Hormone assays were performed on samples obtained on the first day of the mission, just 3.5 hours after launch. This is the earliest time for which measurements of hormones have ever been made during space flight. From Figure 20, you can see that the effect of space flight on plasma ADH was highly variable on FD1 but that it averaged about a 360% increase on that day! Remember, ADH is released in the body in response to high levels of sodium. If you look back at Figure 18, the plasma sodium was at its highest inflight level on FD1. Therefore, there may be a connection here that indicates that, at least initially, the release of ADH corresponds to the increased sodium levels in the blood. Let's look next at the amount of ADH that was excreted in the urine.
Figure 21 shows the inflight and postflight urinary ADH levels as a percentage of preflight baseline values. These data show that the level of ADH that was being excreted in the urine was increased throughout most of the flight. And, in spite of the fluid loading that occurred on landing day, urinary ADH was increased. This may have been due, in part, to the fact that the astronauts drank a special solution containing extra salt on that day. The extra water and salt in the system may have contributed to the body's decreased need for and elimination of ADH. Again, so many factors are operating that it is very difficult to interpret this data without having all of the pieces of the puzzle.
|Figure 21. Inflight and post flight ADH changes compared with preflight values.|
Finally, Figure 22 represents the inflight and postflight behavior of the aldosterone system. Once again, the data indicates the percent change from preflight baseline values for plasma aldosterone levels. Remember, in contrast to body's release of ADH in response to high sodium levels in the bloodstream, aldosterone is released in response to low sodium levels in the bloodstream. Therefore, we would predict an opposite response behavior for aldosterone compared to ADH. In fact, this is what we observe if we compare the general features of Figures 21 and 22. A few hours after launch, the plasma aldosterone was reduced by almost 60% but then it began a slow return to preflight values. On FD8, it was reduced less than 20% and on FD12, it was close to preflight levels. Upon landing, however, the aldosterone levels jumped above preflight baseline values. A major factor to remember regarding aldosterone behavior is that it is highly responsive to potassium levels in the bloodstream. To properly interpret this data would require potassium data and we have not reported this here. We do not have the space flight data for the behavior of potassium, so it is impossible to truly understand what is causing the changes in aldosterone levels in space.
|Figure 22 . Inflight and postflight plasma aldosterone changes compared to preflight values.|