A 30-year-old woman with a long history of chronic pyelonephritis complains of considerable weakness, sleepiness, decrease in diuresis down to 100 ml per day. BP is 200/120 mm Hg. In blood: creatinine - 0,62 millimole/l, hypoproteinemia, albumines - 32 g/l, potassium - 6,8 millimole/l, hypochromic anemia, increased ESR. What is the first step in the patient treatment tactics?