A 17-year-old boy presents with rapid deep breathing, nausea, vomiting, and abdominal pain. He had polyuria, and polydipsia last week. On investigation: Na+: 134 mEq/L ,K+: 5.3 mEq/L, HCo3·: 12 mEq/L, glucose: 320 mg/dL .The patient’s symptoms are due to a hormone deficiency. What is normally the stimulus for secretion of this hormone?