Effects of some naturally-occurring purine ribosides on purinergic receptors in cardiac and smooth muscle

1. Certain marine sponges and dorid nudibranchs contain adenosine, 2-methoxyadenosine, 2′-deoxyadenosine, isoguanosine and 1-methylisoguanosine as free ribosbides. 2. These ribosides have negative inotropic and chronotropic activity on the isolated guinea-pig atria. The decreasing order of activity is isoguanosine > 1-methylisoguanosine > adenosine > 2-methyoxyadenosine > 2′-deoxyadenosine. 3. Adenosine and 1-methylisoguanosine inhibit the spontaneous contractions of the rabbit ileum and this effect is antagonized by theophylline. 4. All these naturally-occurring purine ribosides appear to act on the same purinergic receptors in the atria, since log concentration-response curves are parallel. 5. Theophylline is a competitive antagonist of adenosine, isoguanosine and 1-methylisoguanosine on the guinea-pig atria.