With CA Gruetter, BK Barry, DB McNamara, DY Gruetter, PJ Kadowitz.

In 1998 Ignarro shared the Nobel Prize in Physiology or Medicine with Robert F. Furchgott and Ferid Murad "for their discoveries concerning nitric oxide as a signalling molecule in the cardiovascular system." 

Using strips of corpus cavernosum tissue from 21 male volunteers, the authors showed how the interaction of nitric oxide with the musculo/vascular system of the human penile corpora cavernosa initiated and maintained a penile erection. In this paper they used a drug designated ‘M&B 22948’ which is an inhibitor of cyclic guanosine monophosphate (GMP), to show that this drug has the same vasodilatory effect that nitric oxide has after its molecular interaction with the GMP/guanylate cyclase system on the penis. They determined that this drug was a ‘phosphodiesterase-5 (PD5), inhibitor’ and in doing so, they were the very first to observe the action of a PD5 inhibitor on the erection process. The discovery was commercialized with erection drugs such as Viagra and Cialis.

Order of authorship in the original publication: Rajfer, Aronson...Ignarro.

(Thanks to Juan Weiss for this reference and its interpretation.)