The Na/K/2Cl cotransporter is increased in hypertrophied vascular smooth muscle cells

Hypertrophy of vascular smooth muscle cells (VSMC) is a pathogenic feature of hypertension which may contribute to abnormal vessel tone and function. As a consequence of the increase in cell size associated with hypertrophy, it is likely that alterations in the mechanisms that regulate VSMC intracellular volume occur. Because the Na+/H+ exchanger plays an important role in volume regulation and because we previously observed long term alterations in Na+/H+ exchange and pHi in response to angiotensin-II-induced (ang II) hypertrophy, we studied cell-acidifying mechanisms. To do this, we measured alkaline recovery from NH4Cl-mediated alkalinization, using the fluorescent dye, 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein. VSMC were growth-arrested (0.4% calf serum for 24 h) or hypertrophied (100 nM ang II in 0.4% calf serum for 24 h). Ang II-treated cells exhibited a 107% increase in alkaline recovery over control cells (13.86 +/- 1.87 versus 6.68 +/- 1.01 mmol H+/min/liter cells). The increase in alkaline recovery was not a result of increased Cl-/HCO-3 exchange becaue it was not HCO-3 dependent nor inhibited by 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid. Studies with bumetanide and the sterically inhibited substrate N(CH3)4+ showed that the alkaline recovery was mediated by NH4+ transport via the Na/K/2Cl cotransporter. Ang II-treated cells exhibited a 334% increase in bumetanide-sensitive alkaline recovery over control cells (9.16 +/- 1.90 versus 2.11 +/- 1.46 mmol H+/min/liter cells). Ang II-treated cells also exhibited a 90% increase in bumetanide-sensitive 86Rb uptake over control cells. These findings demonstrate that Na/K/2Cl cotransport activity is specifically induced in ang II-hypertrophied VSMC and establish this transporter as a component of the hypertrophic growth response.