Results from the experiment

The control data and data in the presence of ACTZ are given in table 1.

Table 1 – calculation results

We can critically evaluate these data in the context of the impact of ACTZ, which we know is a carbonic anhydrase inhibitor.  Obviously these data are a from single individual, and if we wanted to do an in depth analysis of the impact of ACTZ then we would have to repeat the experiment several times, building a dataset that can be analysed using statistics.  As we only have an n of 1, then we can only look qualitatively at the results.

Key findings & explanations

GFR – is essentially unchanged, which is what we would expect.

Na⁺ – we can see that the amount of Na⁺ filtered is essentially unchanged, again as expected given the GFR and plasma Na⁺ are unchanged (139 versus 138 μmole/ml – see the  data tables in the earlier chapters).  However, the urinary of excretion of Na⁺ is increased ~3 fold with ACTZ.  As the FENa⁺ is also increased this tells us that the reabsorption of Na⁺ by the nephron is reduced.  Therefore ACTZ is inhibiting Na⁺ reabsorption.

HCO^–₃ –  we can see that the amount of HCO^–₃ filtered is essentially unchanged, again as expected given the GFR and plasma HCO^–₃ are unchanged (25 versus 26 μmole/ml – see the  data tables in the earlier chapters).  However, the urinary of excretion of HCO^–₃ is increased ~15 fold with ACTZ.  As the FEHCO^–₃ is also increased this tells us that the reabsorption of HCO^–₃ by the nephron is reduced.  Therefore ACTZ is also inhibiting HCO^–₃ reabsorption.

Water – consistent with the impact on Na⁺ and HCO^–₃, urinary flow rate is increased with ACTZ, and the fractional reabsorption of water is reduced.  Remember, water follows the Na⁺, so if less Na⁺ is reabsorbed then there will also be a reduction in water reabsorption.

These data are consistent with the effect of ACTZ on carbonic anhydrase. Figure 1 reminds you about the pathway for Na⁺ and HCO^–₃ handling by the proximal tubule.

Figure 1: a model of HCO^–₃ reabsorption in the renal proximal tubule. NHE3 = Na⁺ and H⁺ exchanger number 3.

ACTZ is inhibiting the carbonic anhydrase, which means that the conversion of carbonic acid to carbon dioxide and water is slowed.  Therefore less HCO^–₃ is removed from the filtrate.  In addition, there is an impact on NHE3 (Na⁺ and H⁺ exchanger number 3), meaning less Na⁺ is also reabsorbed, increasing excretion in the urine.  The reduction in transport also reduces water reabsorption.