A 35-year-old man, who for the last 3 years has been on hemodialysis due to chronic glomerulonephritis, developed disturbances of the cardiac performance, hypotension, progressing weakness, and dyspnea. ECG shows bradycardia, 1st degree atrioventricular block, tall and sharp T-waves. The day before he had a serious break from his solid and liquid diet. What biochemical changes are the most likely cause of the described clinical presentation?