K, this is the tidbit that I learned:
Caffeine is a xanthine and it does 2 things that may increase urine output. First, it does cause vasodiation at the AFFerent arteriole. If there is less resistance at the afferent arteriole, then GFR would increase, more blood would be filtered through the Bowman Capsule. The second thing that it does is that, yes, it does inhibit NaCl reabsorption, whether at the ascending Loop of Henle of distal convulated tubule, I don't know (although I would think that it's at the distal convoluted tubule since inhibition at the ascending loop of Henle would cause a loss of potassium as well as NaCl which is not the case). Since more Na is retained in the lumen of the nephron, more water would be retained and excreted as well. Probably it is due to the latter mechanism more than the increase in GFR that makes drinking caffeine makes you pee so often.
Some minor points: 1) vasodilation at the EFFerent arteriole would decrease GFR, but as I said, caffeine is a vasodilator at the AFFerent arteriole to increase GFR 2) Angiotensin II (not renin cuz renin is an enzyme that cleaves angiotensinogen which then forms angiotensin I which is then cleaved by ACE to form angiotensin II) causes vasoconstriction at the EFFerent arteriole. Yes, angiotensin II does cause vasoconsitriction in the systemic blood vessels, but for the arterioles of the kidney, it causes vasoconstriction only at the efferent end as far as I know.