Nonpolar hormones, which can pass through the cell membrane of their target cells, are called lipophilic hormones. Hormones can interact in permissive, synergistic, or antagonistic ways. Some steroid hormones bind to cytoplasmic receptors, which then move into the nucleus. The polar hormones bind to receptors located on the outer surface of the cell membrane.
Many hormones activate adenylate cyclase when they bind to their receptors.
Other hormones may activate phospholipase C when they bind to their receptors. The membrane receptors for insulin and various growth factors are tyrosine kinase enzymes that are activated by binding to the hormone. Once activated, the receptor kinase phosphorylates signaling molecules in the cytoplasm that can have many effects. The release of posterior pituitary hormones is controlled by neuroendocrine reflexes. These hormones are carried to the anterior pituitary by the hypothalamo-hypophyseal portal system.