An analysis of the sympathomimetic activity of 6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (TIQ).

The pithed rat preparation has been used to study the sympathomimetic activity of 6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (TIQ). In this preparation, an increase in blood pressure is indicative of alpha stimulation, and an increase in heart rate is indicative of beta stimulation. Using this preparation, we have found that TIQ is approximately 3 orders of magnitude less potent than norepinephrine in evoking changes in blood pressure and heart rate. The activity of TIQ has also been studied in pithed animals which had been previously sympathectomized with 6-hydroxydopamine. Sympathectomy produced a marked reduction in cardiac norepinephrine, and it nearly abolished vascular responses to injected tyramine. Following sympathectomy, pithed animals were supersensitive to norepinephrine and subsensitive to TIQ. Similar results were obtained on pithed animals that had been pretreated with cocaine. It is concluded that TIQ is a weak sympathomimetic agent possessing both directly and indirectly acting properties. The data are discussed in terms of a proposal that TIQ may be a false adrenergic transmitter.