Objective: The putative “renal-K+ switch’’ mechanism links low K + intake with Na+-dependent Na+ retention and involves activation of NCC in the distal convoluted tubule in response to low K+ intake and suppression in response to high K+ intake. This study used urinary extracellular vesicles (uEVs) to examine NCC abundance and phosphorylation in healthy adults on a high Na+ diet to determine tubular responses to alterations in K+ (in the form of KCl) intake. Design and method: The study design was a double blind, randomised and placebo-controlled trial. Healthy adults maintained on a high Na+ (∼200mmol/day), low K+ (∼60mmol/day) diet underwent a run-in period of 1-week followed by a crossover study, with 5-day supplementary KCl (active phase, Span K, 24mmol thrice daily) or 5-day placebo administered in random order and separated by 2-day washout. After each phase, blood and 24-hour urine were collected for biochemistry, and spot urine for uEV analysis were collected for progressive ultracentrifugation. Participants who met dietary compliance (greater than 80%) and urinary response requirements were included in analyses. Results: Among the 23 (out of 28) participants who met inclusion criteria, supplementary KCl administration (verses placebo) in the active phase was associated with markedly higher levels of plasma K+ and 24-Hour urine levels of K+, Cl- and aldosterone, and lower levels of 24-Hour urinary Na+/K+ (Table 1). Among 18 participants who met inclusion criteria and whose uEV were successfully isolated, KCl intake was associated with less abundant NCC (fold change FC = 0.74 [0.30, 1.69], p < 0.01) and pNCC (FC = 0.81 [0.19, 1.75], p < 0.05) (Figure 1). Correlation analyses revealed an inverse correlation of plasma K+ with NCC (R2 = 0.11, p = 0.05) and delta urinary K+ with delta pNCC (R2 = 0.23, p = 0.04). Ambulatory systolic blood pressure positively correlated with timed urinary Na+/K+ (R2 = 0.08, p = 0.04) during interventions. Conclusions: In healthy adults on a high Na+ low K+ diet, supplementary KCl administration was associated with reductions in NCC and pNCC in uEVs, consistent with the “renal-K+ switch’’ mechanism, and offers a useful target for dietary intervention.