ABSTRACT
Effects of adenosine on K+ and ACh-stimulated contractility and 45Ca uptake were studied in the rat urinary bladder smooth muscle and were compared with those of nifedipine. Both adenosine (10(-5) M) and nifedipine (10(-7) M/10(-8) M) significantly inhibited the contractions elicited by K+ (10(-2)-32 x 10(-2) M), Ca2+ (10(-4)-3 x 10(-2 M) in K(+)-depolarized preparations and ACh (10(-9) M-3 x 10(-3) M). Further, adenosine (10(-5) M) significantly (P < 0.05) inhibited K+ (10(-1) M)-stimulated 45Ca-uptake in the bladder strips. However, it had little effect on inward 45Ca movement resulting from ACh (10(-4) M)-induced stimulation. On the other hand, nifedipine (10(-7) M) significantly (P < 0.001) reduced both K+ and ACh-induced 45Ca-uptake in this tissue. It is concluded that the calcium channel blocking action of adenosine is limited to Ca2+ uptake through voltage operated calcium channels, while receptor operated calcium channels activated by muscarinic receptor stimulation appear to be insensitive to the purine.