Agonist interaction and decentralization supersensitivity in the nictitating membrane of the cat

1. The responses to individual drugs and to combinations of two drugs were studied in normal and chronically decentralized nictitating membranes (NM) of cats. Noradrenaline (NA), acetylcholine (ACh), serotonin (5HT) and potassium chloride (KCl) and the combinations NA + ACh, NA + 5HT, NA + KCl and ACh + 5HT were injected intra-arterially towards the smooth muscle. 2. Chronic decentralization resulted in supersensitivity to each of these agonists when injected alone. 3. The contraction of the normal NM caused by each drug combination showed potentiation in comparison to the sum of contractions caused by the same drugs given separately. The same drug combinations showed no potentiation in decentralized muscle. 4. Decentralization supersensitivity and potentiation due to agonist interactions in normal preparations may share a common mechanism. The involvement of calcium in this phenomenon is discussed.