Figure 1

  Serotonin and dopamine found in the urine are not simply filtered at the glomerulous and excreted into the urine, they are serotonin and dopamine synthesized by the kidneys.^{2, 3} The amino acids and monoamines tyrosine (L-tyrosine), L-5-hydroxytryptophan (L-5-HTP or L-5-HTP or 5HTP), L-3,4-dihydroxyphenylalanine (L-dopa), 5-hydroxytyramine (serotonin), and 3,4-dihydroxyphenethylamine (dopamine) are filtered at the glomerulus and enter the dilute urine of proximal convoluted tubules, see figure 1.

   The amino acids and monoamines in the proximal tubules are transported into the proximal convoluted renal tubule cells. This process is subjected to competitive inhibition when significant amounts of serotonin and dopamine, and their amino acid precursors are available at the transporter.

  The serotonin and dopamine filtered at the glomerulus are metabolized in the proximal convoluted renal tubule cell by MAO and COMT. Transport and metabolism of serotonin and dopamine by the proximal convoluted tubule cells is very effective.  Under normal conditions, only very small amounts of the serotonin, dopamine, and their amino acid precursors filtered at the glomerulous make it to the final urine.

  Amino acids precursors transported into the proximal convoluted renal tubule cells across the apical and basolateral membranes. Once the amino acid precursors are inside the proximal convoluted renal tubule cell they are synthesized into new serotonin and dopamine. The newly synthesized serotonin and dopamine are then transported out of the proximal convoluted renal tubule cell across the basolateral membrane into the interstial space or across the apical boarder into the proximal tubule lumen. The majority of the monoamines transported through the apical membrane into the lumen end up in the final urine. Studies reveal that 87% to 94% of monoamines entering the proximal tubule are recovered in the final urine.

  There are certainly conditions where serotonin and dopamine filtered by the glomerulus do make it to the final urine, such as disease states involving hypersynthesis of serotonin or dopamine (i.e. carcinoid syndrome and pheochromocytoma). But, these disease states are not normal and not a consideration in this discussion.

  Structures of interest are the lumen of the proximal convoluted renal tubule, the apical membrane of the proximal convoluted renal tubule cell, the cytoplasm of the proximal convoluted renal tubule cell, the basolateral membrane of the proximal convoluted renal tubule cell and the interstium surrounding the proximal tubule (see figure 1).

  For this discussion, the newly synthesized serotonin and dopamine in the proximal convoluted renal tubule cells meet one of two fates, they are either transported across the basolateral membrane of the proximal convoluted tubule cells to the interstitium or they are transported across the apical membrane of the proximal convoluted tubule cells to the lumen of the nephron end up in the final urine.

  These two transporters have distinctly different functions. A primary function of the kidney is to eliminate unneeded substances, waste products, and toxic substances from body. Transport of these substances out of the proximal convoluted renal tubule cell is across the apical membrane into the urine is affected by P-glycoprotein which is ATP dependent and affects active transporter in a powerful manner.⁷ The renal basolateral monoamine transporter is an integral part of renal regulatory functions that facilitates transport of serotonin and dopamine through the basolateral membrane to the basolateral boarder into the interstitium.²¹

  The “dual impedance transporter model” developed in this study is as follows. Of importance is the fact that transport of the newly synthesized serotonin and dopamine of the proximal convoluted renal tubule cells is preferentially directed to the tubular lumen via the apical transporter. This eliminates serotonin and dopamine not transported by the renal basolateral monoamine transporter from the proximal convoluted renal tubule cell.^{21, 22}

   It is postulated that there are two internal impedance mechanisms at the entrance to the renal basolateral monoamine transporter, a “serotonin impedance mechanism” and the “dopamine impedance mechanism”. Under this model the functions of the impedance mechanisms are viewed as impeding access to the renal basolateral monoamine transporter or not impeding access to the renal basolateral monoamine transporter by serotonin and dopamine.

  When impedance of access to the renal monoamine transporters exists the phase 1 response is seen on the urinary monoamine assay (see figure 1). If impedance of access to the renal basolateral monoamine transporter exists, the associated monoamine (serotonin or dopamine) will not be fully exposed to the full effects of transport by the renal basolateral monoamine transporter leading to partial access to the transporter by the monoamine. Impedance of access to the renal monoamine transporter leads to increased excretion of the monoamine being impeded by the apical transporter into the lumen and ultimately the urine.

  When impedance of access to the renal basolateral monoamine transporter is present associated monoamine transport is partially blocked and the phase 1 state exists (see figure 1). When in phase 1 increasing the total amount of serotonin and dopamine presenting at the renal basolateral monoamine transporter decreases impedance. With further increase in the total amount of serotonin and dopamine presenting at the renal basolateral monoamine transporter a point is reached where impedance of access to the renal basolateral monoamine transporter no longer exists, this point is the “phase 1-phase 2 inflection point” see figure 1. With decrease in impedance of access to the renal basolateral monoamine transporter in phase 1, less serotonin or dopamine are transported by the apical transporter leading to less serotonin and dopamine entering the urine. Serotonin impedance of access to the renal basolateral monoamine transporter and dopamine impedance of access to the renal basolateral monoamine transporter have never been observed simultaneously at the same time (both serotonin and dopamine are not in phase 1 at the same time). It proposed that there exists a competitive impedance process at the renal basolateral monoamine transporter. This is a state associated with low level amino acid precursor dosing just above the endogenous state. In this state if serotonin and dopamine simultaneously attempt to enter into the competitive phase 1 impedance state of the renal basolateral monoamine transporter the impedance of access to the renal monoamine transporter of one will cease through competitive inhibition of impedance leaving the other’s access to the renal basolateral monoamine transporter in a state of impedance as lower levels of total serotonin and dopamine are present at the entrance to the renal basolateral monoamine transporter.

  Assays of urinary serotonin and urinary dopamine in subjects taking significant amounts of serotonin and dopamine amino acid precursors (in the competitive inhibition state) are direct assays of the serotonin and dopamine not transported by the renal basolateral monoamine transporter and transported by the apical transporter into the lumen and onto the urine. When two assays are performed with the subject taking different amounts of amino acid precursors in the competitive inhibition state, the results can be compared to determine the status renal basolateral monoamine transporter function of the proximal convoluted tubule cells.

  In phase 1, impedance of access to the renal basolateral monoamine transporter exists. In phases 2 and 3, there is no impedance of serotonin or dopamine entering the renal basolateral monoamine transporter. In reviewing urinary monoamine assays (serotonin and dopamine) from the database, the following phase 1 observations were made. In subjects in the competitive inhibition state, we have observed no instances of impedance access of both serotonin and dopamine to the renal basolateral monoamine transporter simultaneously. (I.e. urinary serotonin and urinary dopamine have not observed in phase 1 simultaneously). With administration of lower levels of significant amounts of amino acid precursors, there will be no impedance to the renal basolateral monoamine transporter of one monoamine (it is in phase 2 or phase 3) while access of the other monoamine to the renal basolateral monoamine transporters is being impeded (in phase 1). It is proposed that competitive inhibition of the impedance of access by serotonin and dopamine to the renal basolateral monoamine transporter exists. Changing impedance to the renal basolateral monoamine transporter in phase 1 is dependent on increasing or decreasing the total monoamine (total serotonin and dopamine) presenting at the renal basolateral monoamine transporter. No observations of other methods for changing impedance of access to the renal basolateral monoamine transporter were observed during these studies. It is proposed that in impedance of access to the renal basolateral monoamine transporter, the serotonin or dopamine associated with impedance to the renal basolateral monoamine transporter may not be transported optimally through the renal basolateral monoamine transporter in proper balance due to competitive inhibition by the other monoamine in phase 2 or phase 3 which has full access to the renal basolateral monoamine transporter.

  Phase 2 represents transport of the monoamine through the renal basolateral monoamine transporter with no impedance the transporter process is not saturated. Uptake of the renal basolateral monoamine transporter is very powerful. In phase 2 almost the available monoamine is transported through the basolateral monoamine transporter. In phase 2 very little monoamine is transported by the apical transporter into the final urine. Phase 3 represents renal basolateral monoamine transport of the monoamine through the renal basolateral monoamine transporter with no impedance and the transport process is saturated. In phase 3 as the total amount of serotonin and dopamine presenting at the renal basolateral monoamine transporter increases or decreases, transport by the apical transporter increase or decrease respectively. This is subsequently reflected in the urinary assay. With no impedance of access to the renal basolateral monoamine transporter found in phase 2 or phase 3 serotonin and dopamine experience mutual competitive inhibition in transport at the renal basolateral monoamine transporter.

  Under the dual impedance transporter model, if serotonin is in phase 3 and dopamine is in phase 1 or phase 2, the serotonin in the renal basolateral monoamine transporter, through competitive inhibition, is excluding dopamine from transport through the renal basolateral monoamine transporter lumen. This is also true with dopamine in phase 3 and excludes serotonin in phase 1 or phase 2 from the renal basolateral monoamine transporter lumen via competitive inhibition. The observation is made that in order for optimal renal basolateral monoamine transport to occur, serotonin and dopamine both need to be in phase 3 at urinary levels in proper balance to affect optimal results. High phase 3 levels of urinary serotonin or dopamine can cause the other to be excluded from the renal basolateral monoamine transporter by competitive inhibition, if it is at lower phase 3 levels.

  Under the “dual impedance transporter model”, when two assays are performed on a subject who is taking serotonin and dopamine precursors and a single serotonin or dopamine amino acid precursor is added/subtracted before the second assay, we have the ability to determine the functional transport status of the renal basolateral monoamine transporter regarding serotonin and dopamine transport. In the process the following questions may be answered; Are the serotonin or dopamine in the endogenous or competitive inhibition state? If the serotonin and dopamine is in the competitive inhibition state are they in phase 1, phase 2, or phase 3?

  In all life forms assayed which have kidneys, serotonin, and dopamine show the three phases of response of competitive inhibition, described herein. Life forms studies so far include humans, cats, dogs, and horses.

  State of the apical transporter in renal basolateral monoamine transport is as follows. In phase 1, impedance exits. As the total monoamine levels (total amount of serotonin and dopamine) presenting at the renal basolateral monoamine transporter in phase 1 increase, impedance to access of the renal basolateral monoamine transporter decreases eventually entering into phase 2, associated monoamine transport through the apical transporter decreases as reflected decrease in the urine assays. In phase 2 there is no impedance of access to the renal basolateral monoamine transporter, the monoamine has full access to transport through the renal basolateral monoamine transporter and transport of the monoamines is vigorous. Changes to total monoamine levels presenting at the renal basolateral monoamine transporter in phase 2 on the first and second assay show little change in apical transport and urinary monoamine levels. In phase 3 there is no impedance of access to the renal basolateral monoamine transporter and transport through the renal basolateral monoamine transporter is saturated. As the total monoamine levels presenting at the renal basolateral monoamine transporter in phase 3 is increased or decreased the apical transport increases or decreases respectively and is reflected in the urinary monoamine assays.

  Proper assay is direct assay of the monoamine “functional status” of renal basolateral monoamine transporter of the proximal convoluted tubule cells. Proper assay of urinary serotonin and urinary dopamine is a direct assay of serotonin and dopamine not transported by the renal basolateral monoamine transporter then transported through the apical transporter. By challenging the renal basolateral monoamine transporter with different dosing levels of amino acid precursors which are freely synthesized into monoamines without feed back regulation, the exact functional status of the renal basolateral monoamine transporter can be determined.

  This provides answers to the following questions relating to the functional status of the renal basolateral monoamine transporter that can be used in clinical decision making. Is there impedance of access to the renal basolateral monoamine transporter by serotonin or dopamine? Are the serotonin and dopamine levels presenting at the renal basolateral monoamine transporter adequate and in proper balance to over come impedance of access to the renal basolateral monoamine transporter? If there is no impedance of access to the renal basolateral monoamine transporter is the serotonin and dopamine being transported through the renal basolateral monoamine transporter in proper balance to overcome the competitive inhibition interfering with desired outcomes? Is the administration of amino acid precursors in the proper balance needed to affect optimal transport of both serotonin and dopamine through the renal basolateral monoamine transporter? Are excessive or inadequate monoamine precursors being administered that may inhibit desired results? How is the interaction between serotonin, dopamine, and the renal basolateral monoamine transporter affected by altered dosing of amino acid precursors? Has the subject been compliant with taking the prescribed monoamine amino acid precursor dosing? Has optimal renal basolateral monoamine transporter response been achieved in administering amino acid precursors?

  Statistical analysis of serotonin and dopamine assays prior to starting supplemental amino acid precursors (endogenous state) in comparison with assays performed while the subject were taking significant amounts of amino acid precursors (the competitive inhibition state) revealed that there is no correlation between assays while taking supplemental monoamine amino acid precursors (competitive inhibition state) and those assays performed when the subject is not taking precursors (endogenous state). It was found that baseline assays prior to administration of amino acids have no value in subsequent urinary serotonin and dopamine phase determination of the competitive inhibition state and no value in determining the transporter functional status in the competitive inhibition state.

  It is proposed that two distinctly different responses exist relating to both synthesis and transport of serotonin and dopamine leading to different sets of results being display on serial assay. The first response is the endogenous response seen when subjects are taking no amino acid precursors. The second state is the one of mutual competitive inhibition between serotonin and dopamine that exists only when urinary serotonin and dopamine are high enough for both to display phase 1, phase 2, or phase 3. Defining the phase and status of competitive inhibition is only viable when significant amounts of amino acid precursors are administered simultaneously in the competitive state.

  Baseline assays of urinary serotonin and dopamine in the endogenous state prior to administration of significant amounts of amino acid precursors is an assay of the endogenous state where competitive inhibition in synthesis and transport does not exist. Baseline testing results are meaningless in terms of synthesis and transport when the subject is taking significant amounts of amino acid precursors simultaneously and the competitive inhibition state if that is the focus of study.

  It is proposed that two distinctly different responses occur in synthesis and transport of serotonin and dopamine the endogenous state and the competitive inhibition state.