AXIAL HETEROGENEITY OF POTASSIUM-TRANSPORT ACROSS HAMSTER THICK ASCENDING LIMB OF HENLES LOOP

Functional significance of morphological heterogeneities along the thi ck ascending limb of Henle's loop of hamsters was explored by the in v itro microperfusion technique with special reference to K+ transport. The transmission electron microscopic study confirmed that there are t wo types of cells, with smooth surface (S-cell) and rough surface (R-c ell), respectively, and that the former is abundant in the medullary t hick ascending limb (MTAL), whereas the latter is in the cortical port ion (CTAL). The electrophysiological study revealed that in both segme nts there are two cell populations, one having high basolateral and lo w apical membrane K+ conductances (HBC) and the other having low basol ateral and high apical K+ conductances (LBC). Random cell puncture rev ealed that the ratios of HBC/LBC were 24/7 (77%/23%) in the MTAL and 7 /22 (24%/76%) in the CTAL, suggesting that HBC corresponds to S-cell, whereas LBC corresponds to R-cell. Net K+ transport was determined in two segments by measuring K+ concentration in the collected and perfus ed fluid by ultramicroflame photometry. In all six tubules of MTAL, ne t K+ flux had a direction to reabsorption with a mean of 4.87 +/- 0.46 pmol. min(-1).mm(-1). In marked contrast, in all six tubules of CTAL, we observed K+ secretion with a mean of -3.81 +/- 0.49 pmol.min(-1).m m(-1), The transmural voltage was positive in both segments and was si gnificantly higher in the CTAL (7.8 +/- 0.5 mV) than in the MTAL (2.5 +/- 0.2 mV). From these observations, we conclude that the S-cell corr esponding to the HBC cell reabsorbs K+, whereas the R-cell correspondi ng to the LBC cell secrets K+. The physiological significance of these cell heterogeneities in the overall K+ homeostasis remains to be esta blished.